Just so you know, libimobiledevice can backup iPhones with their idevicebackup utility. It’s CLI only, so maybe not as easy to get into as iTunes but it has worked pretty well for years on my end.
So someone already mentioned this, but there is this list I maintain on Codeberg: https://codeberg.org/RayJW/awesome-foss
There is also this website which is pretty neat if you’re searching for an alternative for a specific software: https://www.opensourcealternative.to/
Organic Maps is by far the best client imho and it has a special outdoor layer when on trails.