Intel needs some special handling to support physical displays with PCI Passthrough; I don't know much of the details, but on "recent" machines, some bits are missing according to : https://lore.kernel.org/all/20260802050824.10554-1-brchuckz@aol.com/

Your write-up is the kind of thing that saves the next person a weekend. One thing worth knowing before you commit to the whole-controller route. The passthrough page has a "Passing through Keyboards and Mice" section further down, and it says XCP-ng ships /etc/xensource/usb-policy.conf with DENY rules for mice and keyboards by default. You edit those to ALLOW, then refresh with /opt/xensource/libexec/usb_scan.py -d followed by xe pusb-scan host-uuid=<host_uuid>. It's at https://docs.xcp-ng.org/compute/ under USB Passthrough. I have no idea whether that covers your USB-to-serial adapter, which is a different device class, and passing the whole controller may still be the cleaner setup for two discrete workstations anyway. Might be worth a mention to @Team-Hypervisor-Kernel on the display question, because that one still puzzles me.