Okay, the direct gave me a lot of gaming anxiety but I think you're right. Even though I'm paranoid about locked content, everything about modern Animal Crossing screams "shortcuts for new players", so I think locking content is against the spirit of New Horizons. I think I will fight my FOMO anxiety/whatever and just start playing tomorrow in the morning like I haven't missed anything, haha. (Seriously, I get random hardcore anxiety about games sometimes and your comment helps give me some persepctive, thanks!)
You will definitely want to start well before the update. It takes at least a week to unlock the Nook shop beyond the bare basics and the musuem, and you'll want the museum ASAP so you can start collecting all the things for it. Don't feel like you have to start RIGHT NOW!! but I would plan to do it at least 2 weeks before the update so you can reaquaint yourself with the game and get all the main unlocks done. At least, if I was planning to restart my island, that is how I would approach it. :P If it wasn't for all the BS grinding I had to do to unlock DIY recipes I would restart my island honestly, haha. Those god damn balloons... And so many zodiac things I missed out on cause my island hates shooting stars, lol.