I seem to recall that the Defold game engine does a really job with older hardware and low spec machines. I've never used it myself, but maybe sometime I will try it out. Also, Godot engine seems to be a little less demanding than either Unity or Unreal, but of course the demands of any engine are affected by its feature set, and Unity has a lot of features that these other engines (not Unreal) don't have. It might be an interesting project to try to make your game using a different engine like Defold and compare the performance results using older hardware. Good luck, that's really neat that you are making new builds of your game and allowing it to evolve and mature!