Good to hear block mode did it.
If I'm reading XO's troubleshooting page right, that 5-minute wait is a setting you can raise: httpInactivityTimeout under [xapiOptions] in a file like /etc/xo-server/config.httpInactivityTimeout.toml, then a restart of xo-server (https://docs.xen-orchestra.com/backups-and-dr/backup_troubleshooting#error-http-connection-has-timed-out).
That path is the XOA one, and I don't know where the ronivay installer keeps its config, so check that first. The page calls it a workaround and says the slow host is the real thing to look at, which could fit your SMR-disk theory, but I haven't tried anything close to a 3 TB qcow2 delta myself.
For the close() error, unless I'm misreading the code, XO closes the source after the failed start and only logs a warning before falling back to a full, so I'd guess it's noise from the timeout and not a second bug.
The stuck tapdisks look more like one for @Team-Storage, and I don't know if a keep-alive during the delta computation is planned.
