If you do it right then you will only do it once!!!!!!
Yep. Bedroom is done, house only has like a 2/3’s basement under it. Office and our bedroom is over a crawl space, barely a crawl space actually, critters could crawl under it but not me for the last 40 years or so. Anyways, remodeled that a few years ago, knew it had a spongy spot or two as at one time it was the kitchen and the chimney for the wood cook stove had leaked. Ended up tearing the whole floor up, then burnt all the floor joists. Paid the hired mans boys to come out and shovel a lot of dirt out from under it, had a huge stone right in the middle, old joists were actual 2x10’s, new ones are treated 2x12’s. ended up cutting a slight notch in a few as that rock was gonna not be moved, ever. Laid a course of 3/4 treated tongue and groove plywood down, then a layer of standard 3/4 plywood. All the joists were placed using a laser, pool could be shot on that floor before the carpet was laid.
Office will be next, guaranteed to find questionable joists as well, will do the treated route again except may go with 2x14’s or 2x12’s on 12” centers to support the weight of a rather large gun safe and numerous fireproof filing cabinets. Or I may just build a mini foundation in the middle of the span so the joists are supported mid point.
If you don’t have the time or money to do it right the first time, highly unlikely you will when it needs done again.