If I understand your reply correctly (the output of free would be clearer), having 80% of RAM used as cache would indicate that it is used for caching files such as map and chunk data. This is normal in Linux, and the game would lag more without this feature. The link I shared explains it better than I can.
If you feel this is still not the case, you can share info such as free output, list of open files, etc, while the issue occurs.
There might be an actual issue in your case, but this feels to me like https://www.linuxatemyram.com/
I’m expecting mapping will write a lot of map data to files in your world folder which might explain the apparent high ram usage. Try looking at disk usage while the issue occur maybe ?
Open source Not […] allow […] commercial applications
For some definitions of open source, this is contradictory (FOSS). Some non FOSS licenses do limit commercial use while making the source available, this could be a possibility for you.
But, I’d instead recommend that you check if a copyleft license such as the GPL provide enough protections for your use case.
I don’t have the information regarding xmodmap, but here are two other options which I am using if you are interested.
I recommend https://github.com/kmonad/kmonad
https://support.google.com/docs/answer/3093339?hl=en