Great question! I haven't gotten that far yet. I have Proxmox running on a 12th gen i5 currently, and my other two compute nodes, a 3rd gen i7 laptop and a 7th gen i5, are currently still serving production workloads, so I haven't had a chance to set up a cluster yet. That said, I don't mind limiting features to lowest common denominator for the k3s control plane node, as I don't need gobs of power for it or advanced CPU features, and I imagine that's the pain you might be facing (though I'm new to Proxmox, so who can say?)
That said, I do have a NAS that I can provide both NFS and iSCSI storage to, so I imagine if I run NFS as the backing storage, I shouldn't have to worry about clustered file systems. Each compute does have local storage, but the plan is to provide that via a CSI for workloads that don't need to be highly available or where fault tolerance isn't a concern.