If you really want to keep those vaults independent and on separate cloudstorages, webdav is the only method working since it allows you to specify the path. This would be possible in the oath2-provider situation aswell, but Enpass has for some reason chosen to used fixed paths.
what you could do, i guess, is to use Folder as the prefered synchronization method. As you probably understand you'd have to use another sync-mechanism such as OneDrive, Google Drive (or whatever they call 'em now) and sync the target folders to the cloudstorage that way, but many people already have them installed anyway.
I don't recall Folder being an option in the Android app previously, but it is present in Android (6.9.0).
So by using something like https://foldersync.io/ on android, you should be able to have as many vaults as needed on one cloudaccount.
While it is a positive thing securitywise to separate the dutys for local decryption from cloud synchronization (and holder of the oath2-keys), it is slightly more complex and possibly carries higher risk of incidents, such as corruption, missed-out sync-intervals having your latest update being written over.
Like @AnakinCaesar mentioned, merging your 5 vaults is alot easier if it's sufficent to store them under one cloudaccount. For instance vault-items previously being stored separately could after the merge be tagged respectively.