This sounds like a neat setup - i used to run a set of 3 old compuets, a Nuc, and old laptops. But they were quite low spec, so i sold them and bought a small htpc instead, quadcore, with 16 gigs of ram and 512Gb nvme.

Also using RKE, it looks like the best cluster 'distro' around.

I used longhorn but found it a little slow, putting it on SSD was a bit of a waste. I tried ceph, it was much faster but i just do not understand how it works, etc. Well enoguh to fix anything if the cluster goes wrong.