It is not clear to me that classic confinement is absolutely required - I would be suprised if it was only a small percentage of users who wanted to use notable to access notes outside their home directory or from external media (and so in these cases for strict confinement plugging the home and removable-media interfaces would give the vast majority of users all the access they would need).
If you need to spawn a shell process to access the clipboard, then you should plug the desktop interface and ship whatever binaries you need inside the snap to do this rather than relying on anything from the host environment.
I understand that launching external editors is only possible with classic confinement however as notable provides it’s own markdown editor, I assume this is the most used interface for note editing and so it is not clear to me that spawning an external editor is core functionality. However, our guidelines for classic confinement do include applications which need to launch external configured commands from the host so this is somewhere in between the two.
Note the making a snap classic does provide a small barrier to installation as there are some users who may not wish to install a classic snap since it does not provide the usual isolation guarantees of strict confinement.
As such, I believe if as the developer and publisher of notable that you really wish for it to be granted classic confinement in light of this, then this can be done to support this external editor use-case, but in my opinion strict confinement would be a better option.