The problem with older GPUs will actually be that GPU’s OpenGL/Vulkan (if it has vulkan) implementation through drivers. If your game is targeting Compatibility then you are already doing the most you can on just your machine, you will have to test on the older machine with it’s likely abandoned drivers.
You could build a export template with removes 3D and Vulkan, this probably doesn’t produce performance increases, nor improved compatibility, only a size decrease. This post goes through reducing the template size for the web, which has similar OpenGL ES capabilities.