My point is a lot of GTX 750 Ti gpu’s can run solely off of the power provided by the pcie slot on the motheboard, some require a 1x6 pcie cable from the power supply. The RX570 on the other hand can often require a 1x8 pcie power connector. Verify that you are using the correct connector, and that it is fully seated.
This comment makes very little sense. If the card is factory stock, it now has 4-5 year old paste on the GPU, which means it is likely dried up and not very thermally conductive. Re-paste it with some good thermal paste. I usually use Thermal Grizzly Kryonaut or Noctua NT-H2 either should work. Thernal Grizzly is better for temps but does not last as long. I have a fairly regular maintenance plan on all my equipment and tend to replace it every two years regardless.
Voltage running high = producing more heat. If you can adjust this down without causing stability issues you should see better performance.
Are you turning off XFWM compositing while gaming. It won’t do it automatically unless you scripted it to do so. Reason I ask is that you mentioned Linux Mint not having this problem, which brought to mind 1) Xfce or Cinnamon. Cinnamon does not have this issue. or 2) Linux Mint has a lot of built in automation, and I doubt their Xfce edition is any different. It may be doing this for you automatically.
Also I would use the lts kernel for initial troubleshooting, once you have satisfactory performance there, then I would play around with alternative kernels. I have tested a lot of different kernels on a lot of different distros over the years, and the headaches that come with using only a non standard kernel are never really worth it. The ROI is minimal and often negative. Always keep lts available, and outside of extremely new hardware the likelihood of a drastic difference with a tuned kernel is unlikely.