Running Stellaris on my secondary monitor set as the main monitor and my MAG274QRF-QD set as the secondary monitor, so far appears to cause no issue whatsoever.
That appears to have helped. I had also turned off Adaptive Sync on my monitor because of an issue with 'total war: warhammer II'. I'll try going back to Borderless Fullscreen soon, but this was already immensely helpful, Thanks.Right click on stellaris.exe , properties, compatibility. Tick "disable fullScreen optimisations" untick every other box in that dialog, Apply and exit.
Then in the launcher Game Settings menu, select fullScreen display mode, Vsync off, then cap the refresh rate at 60.