An answer has been given on the development mailing list:
https://lists.proxmox.com/pipermail/pve-devel/2025-June/071561.html
As far as I understand, having kernel ≥ 6.12 doesn’t automatically mean realtime is enabled. What got merged was the...
updated systemd-boot package will hit the repositories shortly on our end, so that upgrading before removing systemd-boot should avoid triggering this issue.
Bei so hohen Ausfallskosten (welcher Zeitraum?) solltest du dir zum Thema Redundanz Gedanken machen und evtl eine IT Firma mit Planung, Implementierung und Wartung beauftragen.
m2c