Having built one Pi-based KVM-over-IP console and been pleased with it, I built a second for the other rack. The second one is always the honest one. The first build is full of decisions you got lucky with; the second is where you find out which of them were actually good and which just happened to work.
So this is the post-mortem on the first design, written while building the second. Three things I changed.
The first was the HDMI capture path. On build one I used a CSI bridge, which is lovely and low-latency but ties you to a specific Pi camera connector and a slightly temperamental driver. For the second box I tried a plain USB HDMI capture stick instead. It is a little laggier and the framerate is lower, but for staring at a BIOS menu that does not matter at all, and it survives a Pi swap without re-flashing anything. For a console you use to read static text, latency is a vanity metric.
The second was power sequencing, which I got wrong the first time and only half-noticed. If the Pi and the target share any kind of power relationship, a target reboot can brown out your console at the exact moment you need it. On build one I'd solved this by accident with a spare supply. On build two I made it deliberate: fully isolated power into the Pi, opto-isolated control lines out to the ATX header, so nothing electrical crosses between the two machines except signals I chose. No more clever guessing about backfeed.
The third was the case and the cabling. The first box has ribbon cables at unfortunate angles that I am genuinely nervous about every time I open the cupboard. For the second I routed everything with strain relief and a couple of cable clips, and I labelled the ATX header pins on the outside of the case, because future-me will not remember which jumper is reset and which is power, and getting that wrong on a live machine is a bad afternoon.
None of this is a different project from the first one. It is the same project, built by someone who has now made the mistakes. If you are about to build your first, the advice that survives is dull and reliable: use a boring USB capture stick, isolate the power completely, and label the bits you'll forget. The clever low-latency version is a nice upgrade once you know the thing works.