A lot of fun so far ! Thanks for making this

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.

• With the compose key set, you can hit / = to get ≠. Or you can set custom sequences in .XCompose, but I don’t believe you can get the exact combination you suggested.
• Use a virtual keyboard manager such as KMonad

I recommend https://github.com/kmonad/kmonad

Great resource, thanks for sharing