A VPS is already a VM and nesting VMs, even if you get it to work, is generally a Bad Idea™️.
What you're asking for is squarely in "bare metal" territory. Does that reduce your flexibility? Sure. But it doesn't entirely eliminate it. Down the road if you decide you need more RAM or disk those are things you can have added (at a cost). CPU would likely necessitate a migration to a different system so I'd keep that in mind during initial sizing. Also, if you are using proxmox, migration will be as simple as backing up a container/VM and restoring it at the destination.
Your other alternative is multiple VPSes or possibly augmenting the bare metal server with one or more VPSes.
As far as unified billing goes, just have all the services with the same provider. Most providers I've encountered offer both services.
I can't speak to providers in our around Sydney, but I'd recommend checking out lowendbox.com to start your search.