At $DAYJOB, we use the Telmate terraform proxmox provider, too. Upgrades over the years have had a few papercuts (mainly when values are shifted around), but it has overall been an immense timesaver. A terraform repo contains the 2 colocation datacenter proxmox environments plus a local lab edition of the same builds (locallab uses containers vs full VM's on the "real" hardware so that I can run almost all of the configuration on my laptop). Saltstack gets deployed during the buildouts, which picks up the OS/ application configuration after the OS is deployed.

No agentic stuff on our stack, as our security posture can't afford that currently.

As for the load-balancing, I think the later versions have supported targeting proxmox clusters vs a single node, and the newest Proxmox can do resource auto-balancing. That might get you what you need