The issue is that that “main gig” dominates your time. I get up early in the morning to commute to work. I spend 8 hours at work doing what main gig requires. I commute home. If I’m super lucky, as well has having no family or social obligations, and I don’t do any cleaning or chores, I get 3-5 hours to both relax and work on side gigs. Then I’m getting ready for bed or doing one of the other things I have to do to sustain life.
While the person working the other gig full time, while they do a lot of stuff they don’t want to do, all that stuff is in the same vein as the stuff they want to do, and practice like that has value. Maybe you’re not playing beach’s lute suites, but the playing/performing/writing/networking you’re doing has more value to that thing than being a programmer.
It’s not straightforward which one is better if you really want to take the side gig forward and advance.