Trying to use incus copy on a VM with attached custom volumes, I currently run into

Error: Failed add validation for device "volume1": Failed loading custom volume: Storage volume not found

... as also seen in this post. Like in that post, using incus storage volume copy ... to place the custom volume on the target before issuing incus copy for the VM, it results in

Error: Failed migration on source: Error from migration control target: Failed creating instance on target: Cannot create volume, already exists on migration target storage

I'm aware that VMs using custom volumes are not a development priority, but would still like to rely on the simplicity of incus itself for backup purposes. So I tried adding a --exclude-custom-volumes option to incus copy ... and it turned out to work fine for me.
The change is relatively small: main...schnoddelbotz:incus:main

It's obviously not perfect, as the custom volume(s) need to be re-attached on the backup instance - but at least it enables taking a backup of the VM itself.

Would you see this as potential solution (until maybe incus copy includes full custom volumes support)?
If so, I'd happily create a PR for this.