Check your state or country’s laws, you might not even need the contract amended. In the state that I live in any contract clause that tries to prevent you from doing any work entirely on your own time with entirely your own materials is explicitly unenforceable.
Plus if it’s just a small open source library (assuming your employer is sane) it’d be a waste of money for them to even ask a lawyer to write a letter to you, because why would anyone care.
If you really care about getting it right, you can find a local employment attorney and have them explain your local laws and edit and/or negotiate your contract for you. I did that once, but I felt like it was probably a waste of the $900 I paid. (I mean, it definitely was a waste in that case because that job was a nightmare and it only lasted 2 months, lol.)
I just pulled my Bangle.js 2 back out to play with making a better reminder system for myself. It works better than any of the other open source watches I’ve had with my GrapheneOS phone. The hardware isn’t open source as far as I know, but their mobile app (fork of gadget bridge) is, as are all the apps that run on the watch, and (I think?) the watch OS.