The advantage of writing your own "game engine" is that it would only have the exact set of features you need for your game, and it would make the things you need to achieve as fast and as easy as possible.

This means traditional advice for "the basics of a good game engine" doesn't really apply, because if you're just making a bog standard game engine you're wasting your time and making a crappier version of Unity or Unreal.

https://geometrian.com/programming/tutorials/write-games-not...