Interesting project though it might be easier to just have 3-4 Dell Optiplex Micros stacked. I've seen some decent ones go for ~$300 with 9th/10th gen chips and 8-16GB RAM each.
For quieter and less Watt-hungry rackmount in the living room, I ended up moving towards Atom-based ones, currently from Supermicro, with fans replaced with Noctua. Plus a 4U chassis for GPU, with a consumer motherboard (non-ECC RAM).
I recently again almost redid a home K8s production cluster using RasPi 4 8GB (and also looked at products from Pine64 and Turing Pi and others), but product availability is terrible right now. Meanwhile, used x86-64 hardware is easily available, and easily runs official Debian Stable perfectly.