Hello,
first post, please forgive if I overstep or over-read something.
I also am struggling with the customization of the thumbnails,
but I - seemingly (and in the actual state of versions) - found a workaround, using a little bug as a tool:
as I read - and experienced - for some time now the thumbnail is set shortly after an Image (or a thumbnailable attachment) is included into the note and then doesn't actualize itself upon note-editing.
wich means: if you create a new note (or edit a note that does not already have a thumbnail) only add the picture you want to have as a thumbnail, synchronize until the thumbnail is generated, and then just edit the note the way you finally want it to be.
it is inconvenient (and I would love the developers to fix the issue), but right now it works; and if you are on some aspects of your note-taking a little nerdy (like - for example - me 😋) - you might prefer a workaround to an impossibility...
greetings ✨