My main concern is what plants preserve well and what plants might be at risk for breaking down, growing stuff, or attracting bugs etc.
Wow. Yeah. That actually makes a lot of sense. Heaven forbid you use something that'll only take more away from the paper in practicality than would add to it aesthetically.
As for the rest, you really sound like you kind of got this stuff more than half figured out. How long have you been doing it now?