Growing out high undercut Tips? Should I cut it all to the same length or leave the top long?

I've had my hair cropped short like this for a long time . I usually get the undercut shaved to a #1 when I get it trimmed up. My brother is getting married next fall (16 months) and I'd like to do something fancy with my hair for that. My hair grows super fast so I am confident that I can have a reasonable length by then. I am in that awkward in-between phase right now where the undercut is about 2.5 inches long. Should I cut the top to match or leave it? Open to other suggestions as well.