What about OpenStack, or even CloudStack?
PLEASE DON'T DOWN VOTE ME TO HELL THIS IS A DISCLAIMER I AM JUST SHARING WHAT I'VE READ I AM NOT CLAIMING THEM AS FACTS.
...ahem...
When I was researching about this a few years ago I read some really long in-depth scathing posts about Open stack. One of them explicitly called it a childish set of glued together python scripts that fall apart very quickly when you get off the happy path.
OTH opinions on Proxmox were very measured.
I think the main selling point for SME (wtih a small IT team) is that Proxmox is very easy to setup (download iso, install debian, ready to go). CloudStack seems to require a lot of work just to get it running: https://docs.cloudstack.apache.org/en/latest/quickinstallati...
Maybe I'm wrong - but where I am from, companies with less than 500 employees are like 95% of the workforce of the country. That's big enough for a small cluster (in-house/colocation), but to small for something bigger.