Rabbits wont bond, any help is appreciated

Hi, and thank you in advance for your help!
My wife and I have had a boy bunny called Ruth (yes he is a boy despite the name ) for two years now. He is a mini lop, and was neutered about six months ago. He has always been always a bit grumpy, but never aggressive, and usually likes to be groomed. We thought it would be nice to get a buddy for him.
We bought a girl bunny about four months ago, her name is Chise, she is five months old now, and seems to have reached her "final" size now. She has always been very cute, licks our face, loves to be groomed.
First we kept the Chise and Ruth in separate rooms, Ruth in the living room, Chise in the kitchen, they couldnt see each other, but could most likely smell each other. Ruth was very excited, binkied around and such. We spent a few weeks like this.
Then we set a fence between the kitchen and the living room, they could see each other but couldnt touch each other. They seemed to be interested in one another. When we lessened the space in the fence separating the kitchen and the living room so they could almost touch each others noses, there seemed to be a bit of growling, but just very quiet, didnt even sound aggressive.
After a little time we thought that we could try to introduce them in a neutral territory, in the bathroom. There was no aggression, but Ruth seemed to try to mount on Chise, who was rather trying to get away, not running, just moving away, so we ended the date, we thought that Ruth maybe has some hormones left.
Then after a bit of time we opened up the separating fence, so Chise was in her cage, and Ruth could go around in the kitchen. We kept a close look on them so they wont bite each other through the cages fence. No such thing happened, Ruth even lied down next to Chises cage, they were absolutely calm.
Then we did the whole thing the other way around. Chise could go into the living room while Ruth was in the cage. In this case Ruth was irritated, stomping, jumping around in his cage. We thought it would be best to put the cages next to each other in the living room, so Ruth could get used to seeing Chise in that room. This happened quite fast, after one or two days he was no longer irritated when he saw Chise running around in the living room, neither was Chise grumpy when she saw Ruth running around there. We always kept a close look on them so they wont get too close to each others cages.
Then after a time we thought that its time for letting them touch each other through the cages fence. Ruth was out running, Chise was inside her cage. Ruth went to the cage, their noses touched for a few seconds, and the Ruth has bitten Chise in the mouth. It seemed quite bad, but eventually it healed after two days without any problems, without scars.
We thought then that its best if they are kept separately for a little time again, one in the living room, the other in the kitchen as before, and they could see each other through a fence.
Then after two weeks or so, we moved the caged next to each other again in the living room, they could go out, one at a time, saw each other running around, but couldnt touch each other. This has gone for a few weeks. They seemed to be very interested in each other, if one of them ate, the other one began to do the same. When one of the drank, the other one began to drink as well. Same for cleaning themselves and so on. When one of them was out then he/she really wanted to be close to the other, tried to get so close to the others cage as possible. They couldnt do that because we set up an extra fence there (the one we used to separate the kitchen and the living room), so they couldnt bite each other again through the cages fence.
We even took them out to the nearby park in separate pens, so they could see each other but couldnt touch each other.
After several weeks time spent like this now we thought that its time for them to meet each other in the park, which is totally neutral territory. First we set up the pens separately again so they could see each other but couldnt touch each other. There was even a bit of wind, so they were really interested in the many smells they could sense, no sign of irritation or aggression. Then we opened together the two pens so they could walk up to each other, Chise walked up to Ruth and after about two or three seconds without any warning signs they began to fight like crazy, they were rolling around like an aggressive ball, growling, and it was almost impossible to separate them. When I managed to do it (I had lether gloves, but they did not care to bite me, just each other), they immediately jumped at each other again. This took about half a minute till we could separate them completely and close the way between the two pens.
Now it is Ruth who lost quite a bit of fur and has a wound on his back, Chise did not get any wounds. It seemed like she gave back what she got from Ruth the first time.
Now the pens are separated from each other again in the living room. When one runs around, the other one can see it and shows no sign of irritation or aggression, just lies, eats, drinks as usual. The one that is out tries again to get closest to the other as possible, but without any signs of aggression, they just seem to be interested.
During these months we tried the stress bonding as well, with vacuum cleaner. However Ruth has already gotten used to that, it was just Chise who was scared.

Now we are clueless. Despite this long careful process, they were fighting as soon as they could touch each other on totally neutral territory. We dont want this to happen again, we fear that next time one of them will be wounded really badly.

Can anyone suggest something, give some advice?
Or should we just accept that these two bunnies simply cant get along well?

Thanks in advance for any help!

Hi. First of all I would check that Chise is indeed a female. If so, she is now at an age when she can be spayed. If Chise is a boy then this explains why they won't bond as he needs to be neutered.

First of all, as suggested by tonibun, are you sure that Chise is a doe.

First of all, as others have said, check whether you have a male and a female. Then I would get Chise neutered and wait 6 weeks for hormones to subside.

After that time, I would try again in a neutral territory, but assuming that Chise is in fact female, give her a little space to be able to run away. Attempting to mount is usual at first from a male, but won't necessarily be a problem if the female can run away. I would never attempt bonding in a park tbh. There are just too many outside threats and I doubt whether either bunny felt secure.

When you attempt bonding again with them, I would make sure that you have protection for your hands and be in a position where you could separate to prevent serious injury. Rabbit wounds can easily lead to ulcers, in addition also to serious injury.

Let us know how you get on

I agree with the others - Chise needs neutering, leave several weeks (maybe a couple of months) for recovery and for hormone levels to reduce, then try rebonding on totally neutral territory at home. Don't rush it.

I would not even attempt another bonding session until this is done. One is territorial and one is a hormonal adolescent. If they are OK with each other between bars, I would leave them as they are for now, but separate them completely (ie different rooms) if there is any aggression at all. Once they get into the habit of being aggressive with each other, it is difficult to make a successful bond.

The park is not the safest place for pet rabbits - they are being exposed to potential diseases from wild rabbits, to sounds and smells they are not used to, and the presence of predators (dogs, cats, foxes, birds of prey, etc) - all of which will make them nervous and has, in this case, resulted in referred aggression.

Are they both fully vaccinated? (For Myxomatosis, RHD1 and RHD2).

Thank you very much for the help and advice! Yes, I am sure she is a girl. We will try neutering then, and wait a few months. They always try to get through the first "barrier" to reach the other bunny's cage. Do you think this is agressive behavior? Or are they just interested? Otherwise its quite common that they just lie by the side of the other's cage, which as far as I know can be a good sign... or am I mistaken?

Sorry I forgot to answer to one question - yes, they are vaccinated, but as you recommended, we wont take them to the park again. However there are not many neutral territories in the flat, maybe the bathroom, although its quite small, it can be hard to separate them there...

Some bonding advice here :

https://rabbitwelfare.co.uk/rabbit-care-advice/ownership/bonding-rabbits/

I am sorry you are having difficulty and the only advice I have is to put them together again and allow the boy to chase the girl as this is normal and it is their way of sorting out their hierarchies. If there is any nastiness I would shout at them so they know this is not what you want them to do, you want them to get along with each other. The chasing might last for a few days but as long as the female can get away from him, he will get fed up and they should then start to settle down with one another. They have to learn to trust one another. Once over this hump all should be peaceful and you will have 2 happy bunnies. If there is bad fighting then you will have to separate them until after the girl has been spayed.