What would I use to clean the bottom of the tub that is pebbled?

Have you tried a Mr Clean Magic Eraser?

Run some hot water to soften the surface a bit, and then wipe with the eraser, which is now just damp. It may take several tries, but I found it works better than any other cleaner I’ve tried.

Cheap enough too. Even if you have to use a couple of the erasers. Do use the real thing rather than Dollar Store for these first attempts. The others are great for maintenance.

Rinse well.

I have used a handheld scrub brush, a small amount of Bar Keepers Friend Cleanser and scrubbed away until the pebble area was clean.

I would take the easy way out and soak it in water enough to cover the bottom and a 1/2 bottle of clorox overnight. Works for me..

T’s concerns about Magic eraser are likely valid. But since I’ve only used them when desperate, and only bought six in the last 5 years, I’m not concerned. Wear gloves and then rinse really well

I afraid I’m more concerned about the fumes from the bleach sitting in the bathroom overnight, unless it’s closed to “traffic” and well ventilated. I use bleach frequently. it’s a great “ancient” product. But I use it cautiously, around my grandkids and pets. To say nothing of my lungs and I.

Please be careful.

https://learn.compactappliance.com/cleaning-with-bleach/

If you can not clean it. Time to sand and paint. Use a paint that is specific for bath tub. Your local hardware store can help you find correct paint. (Lowe’s makes a great paint as well as Sherwin Williams.)

Home Depot has good paint also. Do not buy the cheapest as it is not as good.

Happy tub restoring!

Here' a post that might help https://chascrazycreations.com/clean-bathroom-natural-products/