Reddit mentions of Scrub Brush Drill Attachment Kits, All Purpose Brush Heads for Cordless Corded Power Drills Impact Drivers, Shower Tile Bathroom Bathtub Sink Hardwater Stains Scum Grout Cleaning Set (Yellow - Medium)

Ajax Powder Cleanser with Bleach, 14 oz (396 g) https://www.amazon.com/dp/B00005UVD9/ref=cm_sw_r_cp_api_i_1LxWDbMHJ0SG1

Use this, wear gloves, follow directions, basically sprinkle it on, toss on some water, give the tub a quick one time scrub (this one time scrub isn’t meant to clean anything, it’s just to lather up the powder and distribute it throughout the tub.)

let this lather sit in the tub for half an hour or so, then come back and use one of these on the tub with your power drill:

https://www.amazon.com/dp/B07D9M667G/ref=cm_sw_r_cp_api_i_8OxWDbYEPS81H
(note this item isn’t available at Lowe’s or Home Depot so order it now and get the shipping process started)

If you don’t have a power drill, you can also use those green scrub scour pads, but man you arm and especially your wrist will be exhausted and you risk hurting or injuring it (ESPECIALLY YOUR WRIST) again wear gloves because this is a dirty job lol.

Anyway rinse as you go, I use a little container of water to help rinse the rest of the tub and other hard to reach places.

You might have to repeat this process a couple more times to get it perfectly clean. There’s also a chance that once you get it all clean that the tub itself has harder discoloration or stains, at that point you might consider just replacing the tub. They can be relatively cheap, even under $300 or so.

source: I just really like cleaning tubs lol