I would argue that the node shouldn’t be in the cluster if its availability doesn’t match the others. If you remove the part-time node, your pvecm concerns go away.

Now, if you have a failure such that the other 2 nodes get restarted, you can manage the VM startups with delays. If one node completes booting 5 minutes before the other, then have the VMs wait 5 minutes or longer before auto-starting. That way, you’ll have your quorum when the VM starts.

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.

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