Been following this series with great interest as I've been wanting to build a 'high' performance k8s cluster at home for ages.
These micros appear to tick a lot of boxes, especially in the physical footprint and power departments. The problem at least for me is finding decent spec machines available that don't cost the earth here in the UK! I am a bit concerned about the longevity of these systems - there's very little space to grow / upgrade over time too, I guess the approach here would just be to add newer more capable nodes to the cluster when they hit the second hand market at the right price, and then retire old nodes once they aren't worth running anymore.
I do think DIY Ryzen builds appear to offer more bang for you buck at the moment (at least here) - Though they take up more space and power as a downside. You need way less machines to get the equivalent compute power - also increases your total number of PCIE lanes as well (so having many NVME and multi gigabit ethernet is achievable if needed).
I think you could build a new 8 core micro ATX Ryzen box (excluding storage) with 5 or 6 M2/U2 ports, 10GB Ethernet, and 64GB of RAM for about £800 - or I can get about 3 decent modern 4c8t micros with 16gb RAM and crappish SSDs for that