No problem but you are going to have to play around with it because its tricky to make this work without experimenting.
At the most basic level a workaround is to leave your monitor always at the 240 refresh rate setting but make the game you are playing the resolution you want.
If you want to take it a step further and force it properly, you can run the game through Gamescope using Steam launch options. This basically lets you run the game at a lower internal resolution while your monitor still stays locked at 1080p 240Hz, hope that make sense.
Quick version;
Install gamescope or I think it comes with “install gamer packages” in Catchy Hello, if not:
sudo pacman -S gamescope
Then in steam find the game you want and put this as the launch option:
gamescope -w 1280 -h 960 -W 1920 -H 1080 -r 240 – %command%
It sounds more complicated than it actually is — once it’s in launch options you just start the game like normal.
Try it and tell me what happens.