Just another note. You have a Hollowtech II external BB. This is still Shimano's current offering, so if there are problems with the unit, you can simply put in a new one. If it's still smooth, Deore-tier cranks on up will slide right in. It looks like Altus cranks and lower are a mix of square taper, octalink (!), and Hollowtech II.

I'm not clear on what's wrong with the current crank/BB. Worn chainrings can be replaced with exact matches, and the BB is still current.