This is a speedrun i'm fairly curious about. Does NG+ mean immediately after the campaign is complete, OR can you start the speedrun at any point after? Like, can i take all the upgrades i have currently on my main profile, and use that for the speedrun, or does it have to be immediately after the Final Boss fight, with only the upgrades from the main run?

You'd be better off asking in the games forums ( https://www.speedrun.com/splatoon2/forum) or in their discord server. As much as we'd like to help, most people here don't run Splatoon 2.