The “gotcha” with filling a pool is properly compacting the dirt. If you just drop a bunch of dirt in there and call it a day without compacting it, whatever you put on top of it will sink over time. If it is just a garden, then it isn’t a big problem (just keep adding dirt) but if you put concrete or tile or footings for a patio, it could be bad. May want to look up the permit requirements for it. They often spell out exactly how to do it.

Better to use concrete (without rebar) than dirt for the bottom part.

Maybe you could get in touch with some concrete contractors, concrete pumpers, etc. and have them dump spare concrete in the pool so they don’t have to clean out their trucks and dispose of extra concrete properly.

I once had a project at my house where I had to remove alot of dirt (about 100 yards over the course of a year). I found people on craigslist who needed dirt and I would bring it to them in a rented dump truck. One guy asked me to drop a dump-truck load in his pool because he was filling it in. OK. No problem. I show up and they had a couch and some furniture and boxes and trash in the pool and I just unloaded the dirt right on top of it. Holy crap. What a riot. I just dumped it and left.

I think it is a great do-it-yourself job. Find people who are ripping out driveways, rent a bobcat and dump truck, go pick up their concrete, charge them $50 a load (they will have to pay to get rid of it otherwise). Bring the stuff to your pool and drop it in. It is cheaper if you can dump right in the pool instead of hand-carting the fill.