I am not sure why the repos suddenly decided to give me Nv550 and Nv555 today but 555 works properly in P6 now. I would have tried it earlier, but I was never given the option until a few hours ago. Perhaps the constant purges and apt clean/autoremove finally cleared the caches properly…
Of course, there was also an update to QT and other components just before that, so that may explain why the drivers suddenly showed up and maybe it was that that fixed the issue, not the driver. I wish now I had tested it before updating the driver.
Anyway, the frame rate cap seems to be gone, there is no screen tearing, and life goes on.