I would say that Free & Open-Source software is a core tenant of solarpunk, and I would prioritize using and supporting it wherever possible. It's a form of prefiguration, and gives us a solid base to build upon that can't be easily ripped out from under us via enshittification.
As for what company you'll be working for, that will be a bit harder to incorporate solarpunk ideals, but not impossible if you manage to find a software co-op that is hiring. Alternatively, if you have enough friends or co-workers that would be interested, you could start your own software co-op yourself! Here's a great video from already successful software co-ops on how they're structured legally, internally, and make decisions.