Jeff Lindholm

I picked Django because we use it at work. But honestly, if you don't know any of them, I would look at the jobs around where you live and learn the one that either has the most jobs posted or if you can look at the trends the one that is growing the number of jobs the most.

All three would satisfy your needs and all three are good choices. Dotnet core would also be a good choice, again if there are jobs in your area for said technologies. If you literally don't care about this for employment, learn them all and just do it in alphabetical order :)

Note: I think you are wrong about Node being too immature - there are also MOSTLY standards for everything you want to do on the backend that have not changed for many years.