Perhaps better advice than "Don't make your Magnum Opus first" is "Don't expect whatever you make first to be your magnum opus, and don't feel disheartened when it inevitably isn't." But also "Don't be afraid to revisit it later."
Because there's really nothing harmful in being passionate about a big idea when you're a newbie, and sometimes biting off more than you can chew is the best way to learn rather than playing things really safe and avoiding challenge.
I feel like the problem occurs when somebody gets really mired in their first creation, which is obviously going to be the one with all your beginner mistakes, and can't get away from it. It's that sunk cost fallacy of "no, THIS ONE has to succeed, THIS is my greatest comic!" Because you don't always get to decide which of your creations is the one that takes off; I know so many people who have two comics, one of them a soulful personal work with deep themes and fascinating experimental art and nuanced characters, and then one of them that's way more popular that's a fluffy romance or slice of life with silly meme jokes or lots of sex.
I don't think of Errant as my "Magnum Opus". It'd be cool if some day people describe it that way, but the way I describe Errant is "The most me comic I'll ever make". It's the comic I feel like "if I didn't at least try to make this, I'd go to my deathbed with regrets." more than "I think this will be the most popular thing I ever make for sure." It's a comic I've been trying to make since I was a teenager that just won't stay in the box. On some level, I've always known this is a great comic, or at least a great set of characters, and the weak link has always been me, so the reason I've revisited it three times over the course of 20 years is because I want to make this idea... but good.
I don't regret my failed attempts at making Fan Dan Go/ Errant, but I think it would have been bad if I'd just kept going with any of those past versions expecting them to become popular based on my passion alone and without all the work I put in outside of them on short comics, other illustration, games work and study. But I think I'd regret never having made them.
Ultimately a big reason Errant happened is because a pro comics friend randomly drew Rekki one day on social media (hilariously unaware it was the day before my birthday. It was apparently just on a random whim):
John Aggs, first place winner of the Rising Stars of Manga, who has worked with Phillip Pullman and all sorts of amazing stuff...drew Rekki on a whim, eight years after I'd stopped making Fan Dan Go... and I thought... "I have to make this comic. If even the crappy, barely coherent, awkwardly drawn version I cobbled together when I was in my early twenties can make that kind of an impact on somebody that amazing... imagine what I could do with it now, with the professional experience I have and all the things I've learned?"
If you need to put your magnum opus in the box for a while, and come back years later, you can do that. Sometimes after a few years or making other comics, you'll think "wow, no, that was an awful comic, I'm never rebooting that" (I feel this way about.... all the attempts at an RPG gag comic I ever made) but sometimes you'll realise maybe teenage you was onto something and it just needs refining and approaching with your more mature art and story skills.