So I just tried to set up MonoGame as a result of this thread, but with a twist: only use non-Microsoft servers as sources, to see if it really is 'open source'.
It's impossible.
I just spent 30 minutes trying to find a single non-Microsoft mirror for the .NET Core dependency. If MonoGame is your Spice Melange then Microsoft's servers are the planet Arrakis, the only source in the known universe.
On Ubuntu you need to add a Microsoft server as a repo, you can't just "apt-get install dotnet-sdk-6.0".
So, yeah, MonoGame is not for me. I'll stick with Godot or SDL2.