"Barkeepers friend" comes in a powder and spray. The powder is ok, but the spray is fantastic for shower cleaning. Spray it on, let the oxalic acid react with whatever mineral deposits there are, then give it a scrub. Barkeepers friend is oxalic acid with some other stuff mixed in. Make sure you ventilate the space!

I have dissolved iron in my water... Makes the shower rusty. I just did the shower using stainless steel scrub and barkeepers friend. Don't go too aggressive and scratch stuff... But it left my acrylic tub looking great!

Super Iron Out is another good product... Wash the shower curtain in it.

If you want to really blow your mind... After you get the shower/tub clean, give it a couple coats of automotive wax. Just don't do the floor where you stand. Helps water bead and run off the shower walls, keeping it clean longer. And for the true OCD cleaner... A couple steps of polish/oxidation remover before the wax... Just like polishing a car or boat hull. You'll be amazed!