Building an eGPU for my MBP

I bought a 2016 13” MBP, having downsized from a 2011 15” fully-spec’d 15” MBP. I find it beachballs all the time, and I assume it’s because I’m driving a 4k external monitor. Would an eGPU help me, and if it would, I have a Radeon video card spare from a PC I’ve raided. What else would I need to make it work? Thanks, Andrew

I don’t see why your monitor would cause beach balls, since you can see the beach balls on the monitor. Is there some reasoning I’m missing? Usually if monitor resolution is stressing the GPU, it just reduces the frame rate, makes the interface ‘glitchy’, etc.

Do you see the BBs when the monitor is not connected?
Can you quantify how often and do a comparison?
Have you tried lowering the resolution in System Preferences | Displays?
Have you run a utility like Clean My Mac to optimize caches and the like?