I don’t think the learning curve will be too high for either option if you’re already comfortable navigating your Draytek’s UI and getting stuff done. Features wise, it looks like there’s a ton of overlap with what you’ve already got with the Draytek.
OpenWRT and OPNsense are both very easy to have a play with - just spin them up in a VM with as many interfaces as you like, and start tinkering.
I probably should have brought this up in the Time to get serious about my router thread, but GL.iNet routers have firmware based on OpenWRT and can be easily flashed with stock OpenWRT too. I did exactly that in the An ode to OpenWrt thread. They’re commonly available for sub-$50, sub-$100, or sub-$200 depending on the desired feature set and would make a very powerful home router or homelab router as well.
For what it’s worth, I migrated my prod Proxmox server from 8 to 9 several weeks ago without issue. Currently running 9.0.10, and it’s been completely stable the whole time.
I’ve not ever used Proxmox Mail Gateway, but have looked into it. As you mentioned the word server, I’ll point out that Proxmox Mail Gateway isn’t a “mail server” in the sense that it is providing user mailboxes, etc., it’s something that sits between your “mail server” and the rest of the internet to do filtering. Given how good PVE is, I’d imagine Mail Gateway is worth giving a go if you’re self-hosting mail - please let us know if you roll it out!