If you don’t know about smart keywords, basically you can set phrases as shortcuts for specific bookmarks. I want a folder with all of my keywords synced, and that folder only. On each profile or account I have have whatever other folders and the placement of the folder with the keywords doesn’t matter as long as they have the same name.
I’m pretty sure you can’t do this. Bookmark sync is all or nothing. However there used to be third-party sync services available that you can consider if it is important to you. IDK if they survived the WebExtension migration though. Also Firefox Sync is really good with End-to-End Encryption and being reliable.