This is neat. I've been doing the same with Pi 1s and 2s for a few years (http://github.com/rcarmo/raspi-cluster) but stopped short of upgrading to 3s and 4s because of heat/power/cost.

I've found that it is possible to run k3s on SD cards quite well if you're patient, but that the Pi 2 master node spent a _lot_ of time just doing internal stuff even on an otherwise idle cluster, so I went back to Swarm.

(I also had quite a bit of trouble with setting up a container registry, which was hard and undocumented in the first few releases of k3s. Ingress was also a bit of a pain, it's much easier to get traefik working in Swarm...)