HA LXC and external storage (CEPH?)

the usual bonding caveats apply (switch support required for some modes, depending on mode you might not see increased throughput since a single connection or even network path will always use one link).
 
Maybe it is best to leave everything as is then.

My switch most likely does not support bonding and if bonding might not even improve performance (I read somewhere it may even introduce additional latency), changing a running system might not be worth the effort.

On top, I don‘t see how I could add a bonded NIC to my config anyway…