Keep shooting your 870 but go have it coated by black ice in Spanish Fork, it will look new again. Run ya about $200.
Couple other points, there are a lot of assumptions that shotguns shoot to their point of aim...nope, only about half do, American rifleman magazine does best shotgun analysis and I have been getting that magazine for 20 years and it proves it with all the shotguns they test.
Have you patterned your 870? Have you done any shooting to fit tests? I don’t mean shoot a cardboard box but actual testing? Look man, everybody likes new guns but nobody tests and evaluates shotguns like they should. I do and what you currently have is good but the rust issue with 870’s is a problem that can be fixed for a quarter of the cost of another gun. You need to truly evaluate your 870 before you go spending money. I can teach ya if you want, shoot me a pm. I just fixed up an old 870, I have a barrel bender and everything. Or go roll the dice and buy a new shotgun, it may fit ya 50/50 chance. Good luck