In case its useful, I did test running compton/picom and no issues right out of the box in terms of refresh rates. Some other issues though so I’d preferably stay on Kwin if possible.
Edit: For anyone looking at this for the same issues, I just ended up switching to Picom and spending the time to get it to my liking. Now my main monitor is a smooth 100Hz and secondary monitor has minimal tearing (more than good enough for that text pages/message/videos that I use that for).