Buns also control mite popluations (which are thought to exist on most buns at normal levels most of the time) by grooming! So a sick bun grooms less as well as gets under the weather immune wise....hence why mite infestations often occur or flare up during ill health of another sort. :wave:
Ivermectin is usually given over 6 weeks at 2 week intervals to break the lifecycle. It can be given in drop form, injection or spray on. Most people go for spot-on or injection, less chance of bun ingesting it then.
I personally feel that as buns probably carry these mites anyway, it is impossible to rid the environment of them completely, instead you just need to reduce the 'load' in the environment in order to give bunny a chance to fight them off, I would just do a thorough disinfecting and quarantine toys etc for a few weeks so that any mites hiding in any nooks or crannies die off, and change/wash all bedding
There's not much more you can do. I expect helping a sick bun to groom might give a little control.