>To your point though, as I'm running my plex server on an old ~midrange laptop, 4K is pretty rough for me to stream as well.

Unless you're doing reencodes processing power shouldn't matter. You can serve 4K video on a 2010s router if you wanted to. If you're doing reencodes, why bother? Download an encode that's appropriate for how you're watching it. 4K for the big screen and 1080p for mobile. Skip reencoding altogether.