I've been using a new program called RetinaRelay[0] to use my 5k iMac as a second display for my M1 Macbook using an ethernet (or Thunderbolt for even better performance). Works brilliantly. It's definitely higher latency than a normal monitor (my ethernet connection is 45ms vs 5ms for a monitor) but it's not noticeable for coding and miles better than remote desktop or similar.
[0] https://www.retinarelay.com/