What were the professional units of the barbarians doing when not fighting? Professional as a specialization that only was concerned with war would be such a small amount. Most of the nobles would still have lands to control and other work to be done even if they aren't out in the fields.
according to Tacitus these "professional" warriors just lingered around most of the time.

the point about all militia/farmer/Kleucheroi units is that they have a sillily high upkeep (higher than regulars) but get relived from it when "working at home"

When did bobbin say that?