As long as you don't kill the card in the process it will not void the warranty. You can disassemble cards these days without worrying about voiding anything. Just make sure you do the research and get the correct size pads for the correct components or you could damage the card or make the performance worse.

Small heatsinks & a fan is best, especially if you redo the thermal pads between the PCB & the backplate.  Mine is pretty extreme, but it looks like this:
 

But all of the VRAM pads are gray (12W pads) now, with heatsinks on the other side... don't have an up to date picture on what it looks like now, but it looked like this at one point: 

 
This resulted in pretty significant iCX Memory Temperature reductions, as well as VRAM & Die temp reductions, before I switched to the HC block.  The list of used things is in the thread I linked above, it's all 1.5mm & 2mm (mostly 2mm) thermal pads, all told if you go that route, you'll be spending somewhere around $60-$100 on pads & heatsinks, and you'll see veyr noticeable temp reductions.
 
And this'll work for pretty much any 3090... the cheaper the backplate, the less effective it will be. The Kingpin has a very rigid & supportive backplate, with a copper heatpipe built in that runs over the GPU die's back area.