I wouldn't be surprised if the OP has probably abandoned the idea of a 5090 now given the stupid prices. But I wouldn't be that stressed about the potential maintenance issues with a GPU block down the line, I think I've only ever dismantled a GPU block once in my time, and that was only because it was a bare copper block that had gone black with that tarnish, and I wanted to see what is was like under the acetal cover. Yeah it was all black but I'm past that being an issue that affects temperatures to any noticeable degree that would really require cleaning. These days they are all nickel plated so that type of issue is not relevant anyway nowadays. It's only if you use fluids with dyes or those awful pastel fluids, or use dodgy tubing with plasticiser, that you probably need to clean it out. So avoid them and he wouldn't really need to worry about taking a GPU block apart.