All historical lessons about early upgrading of systems notwithstanding, I saw the release announcement for Proxmox 9.0 and decided to be one of the bleeding-edge adoptees. I moved my primary workload to my secondary server, ran:

apt update
apt upgrade

Then, update /etc/apt/sources.list, change all references of bookworm to trixie, then run

apt-dist-upgrade

All went well, no issues so far. Already at 9.03...