I don’t have much to add to what I’ve already said. I’m pretty sure Notes app doesn’t use TextKit at all but is built from the ground up from CoreText (just as TextKit is built on top of CoreText).
For your own app, you should write out a list of the features you need and then write a sample app to test out which version of TextKit is best for you - which one allows you to achieve everything you want. If you can use TextKit 2, you should, since Apple says it’s the future. But if you need anything that hasn’t yet been added to TK2 (printing, multiple pages, etc), you will need to use TK1. (Although you might be able to use TK2 for the UI and TK1 for printing.) If you want TextEdit-style built in tables, you’ll need to use TK1 for now (but note they are only available in macOS and not on iOS). Or you can build your own tables from the ground up. I don’t know how feasible that is in TK2 at the moment, as I’ve only ever tried it in TK1 (using custom text attachments for each row).
Ultimately the choice of which is best to use will depend on the needs of your app, and only you can determine that.
Good luck!