What happens if you turn the case sideways (as if it was an open bench test)? Because if temps return to normal it might be the weight of the card (i used to own a 5700xt ultra thicc 3 myself and i know how bulky and heavy they are) is pulling down too much and it actually detached the heatsink from the dye after years of saggining. Try that and see how it goes. If they do go down as they were before it's that. If that's the issue you either unscrew the whole card and after a paste reapply rescrew everything together or you could get by by getting a gpu riser that will prevent your card to sag and therefor having gravity detaching the heatsink from the card. Probably not as effective but a gpu riser is only like 15 euro/bucks. Basically take your case and lay it sideways with the components side facing upwards (basically you should be laying the case on its right side).
For example the 5700xt from asus had all these issues, the heatsink wasn't screwed/mounted on properly, and once you'd plug the card in the pci e slot and let it sag, the heatsink would detach from the dye raising temps and therefor fan speeds. XFX didn't (the one with issues was the thicc 2, but you got the 3 so the issues they had have been adressed in the new model).
If not you can try to undervolting (i hope you are doing that already especially since you're ocing, if not do so please, 1.2v for a gpu it's ridicolously high and unnecessary). That always reduce temps on vega/rdna cards.