Traditional advice would be not to ask for more: that this would be a time to demonstrate your abilities, with an increased chance of assuming that position or something similar in the future. Given that you are looking to FIRE anyway, I assume this isn't interesting? (that is, you aren't planning any kind of second act / side gig where this would be useful experience)
Having said that, will you be required to fulfill your existing job at the same time? If so, then there is something there. What you ask for, depends. You say you are FI, and this is already OMY, so do you have plans for some additional money? Or would you want to ask for help so that this becomes an interesting experience, rather than a burden or outright hell?
continuing with that point, maybe it is time to give notice, and this works out to being involved in hiring / training your backfill--which would be the person doing your job while you are filling in for your boss. It might mean that you leave at the end of the 3 month leave, though. Would you care if it did? That might be a way to make an elegant exit out of it.