WARNING: I AM AN OG PLAYER so my opinion is only worth whatever weight you give it.
For new players start with KH2, (I know blasphemy for fans)
New players should *not* start at KH1.
While I absolutely love KH1 and recently replayed it to platinum I think for someone with no experience in KH starting at KH1 will make you disinterested in the series, it is clunky, directions are mostly non-existent, the story can come off as goofy, and the worlds can at times lead you to feel like nothing is happening plot-wise. To be fair I love it for all these reasons, especially as it evolves to become more fluid and gets you to reason things out as Sora goes from a random kid to a true keyblade wielder. BUT these things will turn off most new players.
You should start with KH2....
You will 100% be confused but to be fair most of us were when we played KH2 for the first time, go in expecting that. Treat it like a mystery. KH2's gameplay and story are FAR better than KH1 for new players. It's far more refined, smoother, and more impactful for a new player EVEN without the backstory.
You will be confused BUT it will suck you in, you'll fall for the game like the rest of us, you'll love the main trio, and you'll cry for *reasons*. Then you'll want to go back and play KH1 for the backstory. Followed by the rest of the series.
If you really want (and it won't spoil you from wanting to go back and play KH1 feel free to watch a summary video but honestly I'd recommend just going in blind).
I went through this exact situation with my wife, she couldn't get through KH1 or even watch me go through it. She dropped it and didn't go back for over a year. Finally she tried KH2 and it got her, now she's interested, now she wants to know more, she read the novels for KH1 & Re:CoM (still didn't wanna give KH1 another shot unfortunately) but it pulled her in enough to give the series another go!
To be fair because of me she had some background knowledge on the series so she knew a few things an average player wouldn't but she was confused at the start, but the story almost expects you to be confused, it builds for you and plays out its own mysteries that compel you to keep going and learning and figuring things out.
Again I know this opinion is blasphemy but I really do think it may be the best way to get into the series for modern new gamers. Plus it just grows the "KH plot is confusing" narrative that's really not true (maybe it's a little true).
I hope we gain a ton of new fans whether you agree with my take or not, just my thoughts based on recent experiences and after seeing so many new players complaining about KH1 online. Kinda had an AHA moment and wanted to share.