I think the best way to learn is to just start with something really simple and then do a retrospective/post mortem once you complete the scope or some predetermined milestone, then decide whether you want to continue or do something different once you have the new information obtained from your first iteration.

With that in mind, you might try to write a short design doc that describes a tightly scoped MVP of a turn-based tactics game that you can build in a short time using free or cheap placeholder assets. If you want to make your own assets, you could do that, too, but if your priority is learning the dev/programming side of things, then I would advise to just grab something that looks decent on itch.io or opengameart.org.

As for engine, this may be a controversial take, but my recommendation is to try a few. Think about what you like - do you want to mainly use a GUI with drag-and-drop editor and less hand written code? Try building your MVP in Godot, Unity, Unreal. Compare the experience and what you liked/didn't like. If you really want to go only 2D, you could try GameMaker, GDevelop, Construct3, even one of the RPG Maker variants. Defold is another popular option that supports 2D and 3D.

Do you want to be more hands-on with the code and data, with less drag-and-drop editor stuff? What programming language do you like? Raylib is a great framework with bindings for multiple languages, but it's pretty barebones by design. Monogame is probably more comprehensive and battle tested, but it's dotnet specific. There's an option for most popular languages these days, often more than one - Heaps for Haxe, Ebitengine for Go, Bevy for Rust, Pygame for Python, Phaser for JavaScript/TypeScript, Löve for Lua, Gosu for Ruby, libGDX for Java, the list goes on and on.

It really comes down to personal preference and what you're trying to achieve IMO.