That pricing page is absolutely abysmal and indefensible, but has nothing to do with what the parent comment said (small one time purchase vs larger recurring subscription). There's a one time payment option (with no price, which is shitty) on that page for v6. That seems like it has exactly the functionality you wanted?

> They have a “one time payment” option. But I thought that’s what I already bought from the App Store when I bought it a couple years ago.

You said yourself you bought GoodNotes 5, and this is for GoodNotes 6.