As far as I'm aware you won't be able to have any app do the same as

mount -t ecryptfs /path/to/Enc-Docs /path/to/Documents

specifically: you can't have an app "receive" writes to a certain filesystem folder in a traditional unix way.

How I'm understanding the documentation, Cryptomator doesn't support exposing the vault as a "Document Provider". The development of that feature seems to still be on hold.
This means you'd have to share, export/import, ... files out of and into the vault, if you want to encrypt or use them.
Though, editing and saving a file "shared" to another app seems to be supported, I don't think it's what you expect or need.

Since I don't have Cryptomator, and am using the following app for this kind of stuff, I'll describe it for DroidFS, which might offer what you need:
You can create an encrypted vault - with an encryption scheme similar to Cryptomator, aswell as one that also hides filesizes and folder structures - which will be saving encrypted data to a folder of your choosing (/path/to/Enc-Docs) on the storage of your phone. You can then sync that folder with Syncthing. I'd probably recommend the scheme which doesn't hide filesizes and folder structure, as Syncthing might have trouble with a lot of pretty small files.

An open vault can then be made visible in the Files app, it'll be listed as an entry after "Android" in the pull-out menu.
It's reachable by holding, then swiping your finger from the left side of the screen towards the middle - you can also use the "hamburger" button on the top left.
For that you'd have to allow the "Unsafe feature" of Storage Access Framework (Expose open volumes, Grant write access) in DroidFS' settings.
note: Storage Access Framework itself isn't unsafe, in the apps context it's labelled as that, as it allows you to let third party apps access the data in the vault - which in turn could save the files unencrypted.

If you now have any modern application, supporting the Storage Access Framework to write or read files, you can choose in the dialogue, in which you export/save/allow access to a folder or file, one from the open, exposed vault.
Files being written into it, edited or read, will be encrypted or decrypted on-the-fly by DroidFS and saved to the phones space only in encrypted form, once·.

if you don't want any traces of 'unencrypted' files on the phones filesystem, you'll have to choose "Export method" "Memory file" in the settings, just below where you allowed Storage Access Framework.
An app accessing the files can still write them (temporarily or permanently) to storage unencrypted though, but with this option it won't be DroidFS, unless instructed to.

Be aware that automatic backups, if the vault isn't opened, likely won't work properly this way, since you'd have to open the vault before apps are able to access it through the "documents provider".

Another caveat: You should also enable "Storage Scopes" for DroidFS, if you don't allow access to all files, and allow access to the folder with the vault in it. Otherwise DroidFS won't be able to properly delete "chunk" files, at least if you're using the encryption scheme which hides file sizes and folder structures.

I hope this is in any way helpful and apologize for (very likely lots of) grammar errors in advance. :')