Going from OpenGL to a game engine with a modern lighting and rendering pipeline, physics, an editor, etc. is a huge amount of work, though. It's worth knowing how it all works and being able to write parts from scratch, especially since what's in a game engine is nowhere near satisfying every use case, but there is also no shame in using a preexisting game engine; if programming and (in the case of a one-man show) designing assets for a game is going to take a lot of time as it is, getting bogged down in game engines would be foolish.