Not that you asked me, but I'd stick with Ubuntu unless you're going to be virtualizing, running dockers, etc on the same machine. Proxmox is cool but overkill if you're just running a node. I have a home services server that I tried proxmox, unraid and other hypervisor-type OS's on for a while. I found that starting with an Ubuntu base and building what I needed off of it is much cleaner and more stable. Less shit to break.

So I want to run a node with a LNbits instance, BTC pay server and relay on this machine. Is proxmox necessary in this scenario?

I have been running Umbrel for over two years on an Ubuntu VM on Proxmox. It's stable, sometimes it runs for months until I reboot after an OS update. However, recently there has been talk of Umbrel frequently sending the server IP address to Umbrel HQ. I am switching to Start9, IBD is running.