All of the “complete” things relevant to real development will be “with direct x”.
Right but it more seems like developing own game engine while it may take more time before even starting a game but it’s always good idea to create own game series based on same core gameplay with own engine, after that it would be more easier to integrate game with custom engine.
Unreal 4 allows create Sub-systems for specific version of unreal framework (C++)
sub-systems are not hard to understand with whole documentation about it, probably studios use thier systems to build any game faster instead of using only default tools in editor.
The best plan for starting is organize the plan.
First, when I had concept of my game I try to sculpt every characters, optimize to low-poly mesh, bake normals and more maps for textures then rig skeletons for them. Get a lot of textures from unreal marketplace and from other resources with static meshes as well, so once I had some assets to work with it I learn about unreal engine 4 framework and it’s source code for basic knowledge how unreal works. Instead of using script and c++ classes I build own sub-systems for gameplay core and after that I start develop game from scratch like story, levels etc…