Oh, one more thing: Straight up ask other vendors about hypervisor support. If I'm having a conversation about another type of product and they tell me they support VMware and Hyper-V, I'm going to ask them what to expect if I switch to Proxmox in a couple years.

The way a vendor answers this won't just help you avoid future lock in, it'll likely reveal a lot about a company's confidence in their product and their support team.