build-your-own-x VS SFML

https://github.com/codecrafters-io/build-your-own-x. You'll find something here. Any project can be made using OOP. OOP is just a programming paradigm or way of architecting your code.

You can take a look at lists like this for project ideas: https://github.com/codecrafters-io/build-your-own-x

I would also say when learning Unity, you will learn much more Unity-only stuff and less C# stuff. If you really want to learn C# by making a game you can try a less complex framework like Monogame or SFML. Not that those don't have quirks and such but you are forced to do everything in code and they don't offer all the fancy assets and systems. It will take much longer to make a game but I think it will be more helpful for learning C#.

SFML but I don't really recommend it, I don't have a strong alternative though. It's half-baked in a lot of it's implementations and development has all but died on it.

Learn how to implement a GUI, you will need a library like this one: https://www.sfml-dev.org Learn how to print stuff in screen, emit sounds, music, etc.

You can also look at SFML's implementation of a UDP Socket

I don't thinks so, but you can try sdl, sfml or learn any graphics libary (I would suggest opengl)

SFML is a good option; it's well-documented and not too difficult to learn.

But since your goal is "learning C++ by making a game" rather than "delivering a game" SFML may be the way to go. Pretty much everything you learn through SFML should be transferrable to other C++ jobs as well (sans Embedded Systems.)

It sounds like all you are interested in is a 2d graphics library? If so, there's tons for every language. SFML supports C++ for example.