It's a shame much of the skills developed in Unity don't really transfer elsewhere. I'm sure some of it can be transferred to another engine like Godot, but most of the gotchas and subtle things you pick up after years of experience won't transfer.
It's why I like to stick to more fundamental tech when building something... at least the skills you develop will be with you forever.
My goto has always been C and SDL2. I have always been able to create the games I want to make with it and it's nice to be able to transfer skills from one game to another (and often can bring code over from one to another).
I don't recommend building a generic engine or anything.. I start with putting a window on the screen, capturing player input and from then on I focus on the core game itself. The entire code base is the game, there's no engine unless you consider the game an engine.
Would I recommend this for building a AAA game and targeting 12 platforms? Probably not. If you're dabbling with game development then I think it's an incredible way to learn a lot while building up a skill set that is transferable between projects.. plus you won't get the rug pulled out from under you like Unity just did :)
I would also add LuaJIT. Being able to easily extend C types and do things like wrap vector math or string operators around SDL types without the complexity of C++ is a treat.
YMMV, of course, a lot of people have issues with Lua and LuaJIT is pinned to an old version of the language.
It's why I like to stick to more fundamental tech when building something... at least the skills you develop will be with you forever.