Did it a second time on a separate worker node. Worked flawlessly, at least the migration itself.
I discovered that chmod 777 and mounting the external drive with context=system_u:object_r:container_file_t:s0 was mandatory.
I’ll report my findings regarding the post-migration steps. I had some problems at first try that are discussed here Nextcloud error after migration from NS7 - #20 by pagaille
Thanks again @mrmarkuz for the good pointers and insight you shared with me.