There's a minimum number of days off you'll get flying for an airline. Typically it seems to be about 12 per month. If you can fit a full time job into those 12 days, go for it. But your airline employer will expect you to prioritize them first.
Sidebar question: I don't understand airline pilots working another job for additional money. If you want more earnings, why wouldn't you just pick up additional flying at the airline? It's gotta pay more than any side-gig you could get.
