If you've already started learning XNA then sticking with that would probably be a good option!
Yes, Microsoft have stopped developing it, but that doesn't magically stop the existing framework from working -- it's a relatively mature and capable framework which can be used to make some great games, and there are plenty of learning materials and other programmers available. XNA will no longer be updated to take advantage of newer technologies, but that shouldn't be a problem for a 2d turn-based PC game, and your learning experience will be largely applicable to MonoGame (and even slightly applicable to completely unrelated frameworks) for future development.
If you really want to move away from XNA to something actively supported MonoGame is probably a great option for you, but I would honestly recommend you just stick with XNA for now.
Hope that helps!