I’m glad I read this thread. I had already made the change for i915.enable_rc6=0 but was curious to check whether my Librem 15v3 also showed enable_psr=0. Following @Kyle_Rankin’s hint, I found systool (which is part of the sysfsutils package) and ran the command listed above. I can confirm enable_psr = "0", but enable_rc6 is still set to "1", despite having run the steps in the linked workaround. Powered all the way off and on again, but still enable_rc6="1"
Finally I ran sudo update-grub, then rebooted and now enable_rc6="0". We’ll see if this helps!