You’ve had this problem from the beginning, right?
Please restore the initial state (remove the nomodeset parameter, uninstall plasma-x11-session and kwin-x11, and use Wayland), then test whether the issue occurs when only one external monitor is connected (then with two and then with three monitors).
Because only the Intel iGPU is active. For applications that are supposed to run exclusively on the Nvidia dGPU, use prime-run. But the 100% utilization of the Intel iGPU plus that:
suggest that the actual cause lies with the Intel iGPU. The next time you encounter this issue, run this in the terminal:
garuda-diag
and post the generated link here (or better yet, upload a complete journal to “Garuda's PrivateBin).
At the moment, it’s just pure speculation, but you can take a look at this topic for more information: