Thats very helpful, thanks so much. I will take a read!If I'm understanding correctly what's missing for you is the folder preview which, although technically a thumbnail, is not a thumbnail regarding other settings. The thumbnail parameter to which you refer in the bookmark advanced settings controls thumbnails of the individual files; Similarly there is the View > show thumbnails setting. I don't know if those are prerequisites for the folder preview but it makes sense to leave those enabled until sorting out the folder preview
Coincidentally the developer may be working on a tweak for folder preview over SMB share in recent beta versions, in response to a recent request which you may be able to find in this thread and the main thread. The user that requested recently reported that it was working. You may find those posts by searching this thread and the main thread for recent posts mentioning folder preview and SMB share. See my signature for tips about working around XDA search engine glitches, desktop browser recommended.
[EDIT] Here is the confirmation that folder preview over SMB seems to be working: https://forum.xda-developers.com/t/...y-featured-file-manager.1523691/post-88031353. You can find out more of the history of that request by reviewing that user's recent posts. There are also some posts from last year from someone who was seeking to do the same thing but it may not be worth digging into those if the new fix works for you.