A master's will check another box and enhance your score.
I'd say that relevant aviation experience is more important than a master's degree. Don't delay upgrade, LCA, other instructor, etc for a master's.
As far as union jobs, yeah that's good stuff and will come out at the interview but I'm not sure there's always a box to check, and that's what gets you the interview in the first place. The masters always checks a box.
Plus if you ever need a new career, you'll generally get paid more for having a masters and it usually doesn't matter what the major was.