ATC cannot be a backup or side job. The training and currency requirements are extensive and there is a maximum age of 30 to start (unless you are a retired military controller). It could be an ejection-seat I guess, but only once if you decide you hate your regional job before age 30.

ATC hiring has almost nothing to do with airline/pilot hiring cycles...it seems to have more to do with demographics (retirements), policy, federal funding, and politics.