How to remap a regular key into a modifier key (Shift...) in Linux

On my new laptop keyboard, there's a strange key next to my shift key. Frankly it's useless to me and having a shorter shift key doesn't let me reach some of the combinations that I was used to. I'd like to turn it that key into a left shift as well so that I have one long left shift again. First I've tried xmodmap according to this answer , and same as him, I've had no success. Then I tried to create a new xmodmap config by copying the entry of the left shift key into the strange key's row. Applying the settings however took very long (>10s) so having that in my bashrc was not an option, since it made opening new terminal tabs take way too long. It also didn't work in X either, only terminals. I also read that xmodmap is mostly considered outdated. Lastly I tried to use xkb but frankly couldn't really figure out how according to entries on the Arch wiki and here.