I don’t think you can do this without a block-based approach. That part isn’t super novel, for example I wrote textbook editing software circa 2014 which worked largely the same way.

What notion has done that is impressive IMO is made the editing experience enough like the MS-Word “inline” editing that people seem to understand it and use it without minding that it’s a block editor. The project I worked on back in the day wasn’t quite as slick, and there were a lot of users who got frustrated when things like copying text across multiple paragraphs didn’t always do what they had in mind (imagine a text selection from the middle of one paragraph (block) to the middle of another - that’s complicated to deal with).

Nice to see Notion take this model mainstream with such a nice UX.