You are asking 8USD/month for sync and versioning, which is not unreasonable, but …
I’ve been using a wiki as my brain for over 20 years. Some key points I’ve learned are:
- Do NOT have multiple Vaults, because you are going to be having duplicate information between them or have to link between them anyways. Vaults should be namespaces.
- Do have folders where you can bundle notes with relevant attachments. If an attachment is share between notes, it can become its own note with links to it from each usage. I have been able to implement this, but I mention it in relation to Selective Sync.
- Selective Sync is inevitable! My note structure should not be forced upon someone else, but I should be able to easily share pages, folders or large parts of my vault/database with others e.g., via tags or namespaces. And those shares should easily be added to my Vault, perhaps under a separate namespace. The vault size will continue to grow and you are not going to want to share/sync all of it, all the time e.g., to a smaller devices or to colleagues.
- Management tools are needed for refactoring notes into namespaces, folders and tags. The possibility to Archive/retire parts of the vault should be possible.
- versioning is not optional and should be per note.
I also think you should consider splitting the sync into different price categories according to usage e.g., 3USD for a small amount of data synced, 8 USD for larger amounts, and more for larger vaults.
I have a hard time subscribing to your sync service when I know down the road my vault is going to be too limited (as in just a project or work) or too general that I can’t share all of it with work or friends.