“There is no way your engine can be as good as any of the commercial ones already available”.
That’s wholly incorrect. The best engine for the game that you want to create is the engine created specifically for that game. You don’t need to implement all the features of an engine like Unity, because the chances of any one game needing all of them is pretty low. The engine created for your game will likely be a bad engine for any other game, but not for your game.