I'm running Wayland. I do feel that Plasma is using my iGPU to render the desktop since it's quite noticeable some stutters and lower performance compared to disabling the iGPU and having both monitors on my dGPU, but unfortunately I can't really chose what gets rendered by what. On Windows, this setup works fine, I can chose Firefox to use the "power saving" or whatever and it runs on my iGPU, videos get decoded by it.
I tried plugging my monitors on my motherboard (I have a HDMI and DP outputs) and it works as expected, everything renders on the iGPU and I'd need prime-run for my games, though this is far from ideal since I lose VRR and HDR.