As someone who has been looking to lessen my reliance on all things Microsoft, the hardest thing for me to quit has been C# and .NET. It's just a great combination of developer experience, speed, and features. It's also probably the least problematic part of Microsoft to continue using, but it still doesn't feel great.
dotnet has gone pretty open source these days, no? I guess it may depend on the rest of your environment - maybe the Windows specific stuff like forms and WPF are still under them?
Other than that, I think you don't need too much dependence on them, depending on how you look at them controlling it. It being OSS personally gives it more stability since it can always be forked if they pull some nonsense.
The tooling isn't all open source. VSCode C# debugger isn't, for example.
I am on the same boat but I'd say it does feel great since .NET belongs to .NET Foundation and is open source. Its development is very transparent too.