Fill the tub with hot water and oxyclean to soak overnight? That has worked for me with grimy “non slip” areas but this seems more serious, clearly.

It looks almost like either the finish was damaged or water seeped under the finish from the drain area and separated it from the base. Either sound like how it looks in person? If it really is mineral stains, this stuff will remove it:

You might be able to get a free sample from Ace if you have one near. That's how I discovered it.
https://www.bio-cleanproducts.com/ace-hardware/

When I moved into my apartment my whole tub looked like that, my aide was determined to clean it, so after applying some spray bleach cleaner and comet and letting it sit for a few minutes, (you can't let the bleach dry) she got all the stains out, it took her weeks but today my tub looks brand spanking new. She used a soft scrubbie so she didn't scratch the tub.

Yes, spray a decent amount of the bleach, then sprinkle the comet,