My alliance for the first time is going to finish round 8 in this Galactus event. What happens next? Can those players who haven’t reached full progression still keep playing against him, or are they locked out?
You can keep playing.
Even after you reach top progression, you can play the side nodes for 10xp and 1000 ISO. The actual Galactus fight becomes pointless after you get 8/8 rewards.
But Galactus can still be fought, it appears.
Sure, but you get nothing out of it. I usually just take 3 random guys, hit him once and then retreat.
Get the iso/xp from the side misssions, get Galactus to 8/8 for xp, retreat Galactus to play the next side mission