Your situation, IMO, is one you should be especially careful of. You have some background to draw upon. It was awhile ago, though (how long? you say “ages” but what does that mean? 1yr? 10yrs? 20yrs? recency is relevant). You should be especially reflective about each lesson to make sure you’re not skipping over some detail that might be important later. Someone who spends several sessions on a single lesson repeating it many times is less likely to miss things like that. But for you getting to a lesson, feeling like a lot of it is familiar, and you blaze through it saying you can still do it, it’s going to be a lot easier to miss stuff.
Obviously, these are self-guided lessons and ultimately you are the one who makes the decision regarding when to move on.
But keep this in mind - going too fast risks harming your progress by missing things that you’ll have to go back and address. When it comes to some techniques, there’s a good chance that you’ll even have to UN-learn some things before you can really learn the “correct” way that you can then build upon for later skills. unlearning things can be harder than learning something from scratch. I am a skills coach in a different activity and these are truths that are essentially universal.
If that’s the risk of going too fast, you do also need to be aware of the risks of going too slow. Mainly the risk of getting bored and/or growing stagnant in your development. They both have their traps, so you need to find balance between the two.
For playing guitar, Justin makes a good point when it comes to “consolidation” at the end of each grade. That’s your opportunity to take the material from the lessons within that grade and to apply them to different songs in different contexts to ensure that you’ve got them all pretty well dialed. IMO, this is where the meat of the lessons really lies.
Now, if you STILL think you’re ready to move on, then go ahead. It IS a self-guided course and there’s nobody actually standing there ensuring you can do certain skills well enough before you are given the materials for Grade 2. If you went too fast, you’ll realize it.
Everybody’s path is a bit different. I stopped at the end of Grade 1 for a long time working on songs. Mostly so I could develop my ability to hear and find the rhythm in a song while playing in a group setting. I definitely didn’t stop learning new things. On the contrary. I reached a point with my own playing where I had immediate goals outside of Justin’s grades. Maybe some of those were things Justin covers later. But they were priorities for me. Along the way I started picking up other things, too. New chords (my chord library is WAY beyond where I am in Justin’s grades at this point), rhythms, riffs, etc. I only just started Grade 2 recently. But I’ve absolutely learned quite a few bits and pieces within (and beyond) it. That said, I’m still working my way through it so I can make sure that I’m not skipping over some skill/technique.