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For reference - I have gotten the same result on multiple different hosts

last time I've checked, k3s worked with built-in flannel disabled and installed separately (you can still use etcd bundled with k3s as datastore, just need to mount a couple of directories from rancher's datadir), afterward everything else should fall into place

Oh - so I need to install flannel separately?

You might also look at https://docs.k3s.io/advanced#additional-os-preparations I've had to use the

--prefer-bundled-bin

flat in my environment due to using an Ubuntu cloud OS that did not come with IPTables/NFTables (and the flag avoided my having to add the packages to my Cloud Init config). But that was just my own experience.

Appreciate the help, everyone! When I disabled the internal flannel network, installed flannel separately, then reinstalled multus, everything worked great.