A couple that haven’t been mentioned.
Albert’s Schloss - really good vibe, always has a crowd of all ages, regularly have live bands and singers
Impossible - deffo recommend if you like Gin as it’s a gin bar, again gets a good crowd of all ages
Mojo - more rock’n’roll than Indie
Good bars in Northern Quarter - Affleck & Brown (typical Brit Indie music. Very Oasis-y), The Pen and Pencil, The Shack, The Fitzgerald, Walrus, Crazy Pedro’s (they do AMAZING pizza)
Also, Chorlton area of Manchester has some great bars. About 15 minutes in a taxi from the city centre if you’d like to venture out somewhere less busy.
& finally I would definitely agree with above poster - FifthAve nightclub and 42’s are probably the most Indie places in Manchester, however will be full of students and underagers with fake ID

AVOID
Enjoy your gig- Manchester is a great city!!