I have 2 nodes and a raspberry pi as a qdevice.
I can still power off 1 node (so I have 1 node and an rpi) if I want to.
To avoid split brain, if a node can see the qdevice then it is part of the cluster. If it can't, then the node is in a degraded state.
Qdevices are only recommended in some scenarios, which I can't remember off the top of my head.

With 2 nodes, you can't set up CEPH cluster (well, I don't think you can).
But you can set up High Availability, and use ZFS snapshot replication on a 5 minute interval (so, if your VMs host goes down, the other host can start it with a potentially outdated snapshot).

This worked for my project as I could have a few stateless services that could bounce between nodes, and I had a postgres VM with streaming replication (postgres not ZFS) and failover. Which lead to a decently fault tolerant setup.

Please do add a tag to your post as stated on the sublemmy sidebar! Thank you. :)

You can use a small device like a Raspberry Pi as a Qdevice to be the third vote in quorum. It doesn’t have to be a full Proxmox server.

You'll need a QDevice to keep consensus. That wiki article will cover how to set it up and some drawbacks to QDevices. You should be able to run it on a low-power device like a Pi to keep the cluster going.

AFAIK forcing it to a lower number is fine if you're not doing HA. I remember reading something along those lines on a forum, but I could be remembering wrong.

If you're not using Ceph or HA, then I don't think there would be any negative effects from not having all the servers in the cluster ready.

If you are not using any HA feature and only put servers into the same cluster for ease of management.

You could use the same command but with a value of 1.

The reason quorum exist is to prevent any server to arbitrarily failover VMs when it believes the other node(s) is down and create a split brain situation.

But if that risk does not exist to begin with, so do the quorum.

I haven't tested this at all, it's just popped into my head, but, could you create a VM on one of the nodes and join that to the cluster?

If it does work, I wouldn't recommend it. But I'd be curious to see if that would work.