As you are providing unlimited hay - this is very unlikely to happen unless bun is unable or unwilling to eat hay due to early diet experiences or dental issues.
I do not feed pellets at all and my buns do not starve!
The best diet for a rabbit is one which mimicks as wild a diet as possible. This means unlimited amounts of indigestible high levels of fibre which are low in protein such as hay/grass, safe bunny weeds and veggies/greens, and much smaller amounts of high level carbohydrate and protein such as pellets and fruit.
Generally, unless bun is elderly or ill or unable to maintain a healthy weight (your vet should be keeping a weight chart on their computer ideally) rabbits really only need an eggcup full of pellets once per day, whatever the size of the rabbit! Smaller rabbits they say need more energy per bodyweight than larger rabbits. If you feed both your buns an eggcup full of pellets this will be plenty.
The rest of the diet can and should be unlimited hay/grass and I personally see no reason as to why buns can not have a good cupfull/small plateful veggies once per day - but introduction to these must be done carefully over many months to allow the caecal flora in bun's guts to adjust correctly. It is worth remembering that some rabbits are particularly sensitive to certain veg, this is why new veg should be introduced once every 48-72 hours and if excess caecotrophs are produced then you either limit or remove this food from the diet.
In my experience excess caecotroph production is due to a diet too high in carbohydrate/starch - basically too many pellets. :wave:
ETA any dietary changes must be made gradually, this includes cutting down pelleted feed. Withdrawing or reducing pellets suddenly if bun is not used to grazing on hay/grass (or sudden introduction of grass if bun is not used to it) may cause bun to get poorly.