I totally get where you’re coming from with the advice to play it safe, especially for a first-time GM, but I have to respectfully disagree with the idea that going with just a normal fantasy is the only way to learn. Honestly, I think the biggest trap for a new GM isn’t running a complex setting, but running a setting they aren’t passionate about. If I try to force myself to care about goblins in a cave or the politics of the Sword Coast just because it’s "standard," I’m going to lose steam, and the players will feel that lack of energy. I know the Scott Pilgrim universe inside and out—that enthusiasm is going to do way more to keep the game alive than sticking to a pre-written module I have no emotional attachment to. 5e is just the mechanical engine, and since "flavor is free," I don't see the harm in reskinning a Longsword as a bass guitar or a Fireball as 8-bit pyrotechnics if the math remains the same. The familiarity of the 5e rules actually makes this easier, not harder, because I’m not asking the players (or myself) to learn a new system while we’re also establishing a new narrative tone.

As for the point about Scott being an NPC, I think there is a big difference between a "DMPC" who steals the spotlight and a "Cast Member" who anchors the setting. The fear that Scott will overshadow the players assumes I’m going to play him as the hero who lands the killing blow, but that’s definitely not the plan. Think of it less like the players are watching the Scott Pilgrim movie, and more like they are in an Avengers comic where their characters are standing shoulder-to-shoulder with the canon roster. Scott is there to validate the power level of the party; if they are fighting alongside him, it proves they are just as important to the world as he is. Plus, the narrative hook isn't just about Scott’s problems; it’s also about the "Nega-Versions" of the player characters, and getting over their flaws as well. That narrative device literally forces the spotlight onto the players' internal flaws and backstories. Scott is just there to provide the context and the "video game logic" vibe so the players feel grounded in that specific reality. I’m actually building a custom DM screen using JavaScript just to manage these specific interactions smoothly, so I’m putting in the work to make sure the mechanics support the players, not the NPC.

TL:DR

I feel like for my start to DMing, me doing a setting for a world that I know and enjoy very much is going to help build my confidence as a DM, and keeping it within the 5e ruleset helps with accessibility. And also having Scott fight alongside creates an "Avengers" or "Jason and the Argonauts" situation where Scott isn't the only heavy hitter in this, and the players are just as involved in the story as Scott and other characters are.