fiberglass tub sinks around drain

Sounds to me you have a really cheap tub, or else someone using it is way overweight. If the drain flexes, you will eventually develop a leak in the pipes.

Solutions are to either get a better tub, or go on a diet. Or just live with it until something breaks and causes a flood, which could lead to more damages to the house. I worked as a did plumber for years and I never saw a tub flex like you explain, nor did I ever see a tub with shims or spray foam under it.

Apray in the foam and fill the tub to keep it from lifting.

I've seen several acrylic tubs installed with high density spray foam AND shims.

They must be selling some real crap tubs..... That was unheard of in the past. There are so many building codes these days, and some of them are darn near senseless. Yet they allow crappy tubs like that.... Dumb, dumber dumbest....

I'd hate to have to try to repair the drain pipes on one of them, and even worse, try to replace one of them tubs.

This proves one of the things I always say. OLDER IS BETTER!

Yah, foam will support up to 300 lb but for your typical 500+ lb democrat you'll need a bed of concrete and rebar.

Excellent videos.

I don't personally know what type of filler is usually used to support underneath a tub, but I guess that spray foam is one option that people use.

It also appears that your tub may be on a ground level or concrete floor so you may not be able to access it from underneath.

But, given what you wrote and what the videos show, I think if it were me I would open the wall up again and apply more of the same type of spray foam under the drain area that was used around the sides. It seems like it just needs a little more support in that area. I would try not to overdo it and do it in stages so the foam doesn't lift the tub up or cause it to buckle up in that area -- I would just do it in stages, a little at a time.

I also don't know whether it would be better to fill the tub with water first or not. My inclination would be to fill the tub and then do the spray foam with the tub full.

These are all guesses on my part but maybe others here will know better than me what would work best.

Good luck.

Lay a layer of poly on the floor before foaming it, and the tub comes out like butter.

That's the only safe way to do it. Don't want the foam jacking up the tub Make sure you leave an escape rout for the expanding foam in case you happen to slightly over-fill it.