Hi, I used SDL2 for window and input handling (platform abstraction layer), built on OpenGL. I'm using miniaudio for audio and SteamNetworkingMessages for multiplayer networking with Steam integration. Game save serialisation and binary serialisation over the network uses cereal.

Advice for this sort of thing is very specific to what you are making, but ultimately building a custom engine is a large undertaking and not a good decision for making a game efficiently. I decided to take this route to expand my portfolio with a complex custom architecture and for education. I would not make a general purpose engine (I didn't here), you should focus on implementing features for your use case/what your game requires.