I’ve spotted a few good and useable password managers which are open source and can be self-hosted:
Maybe you provide Passman with NextCloud but I have no idea how good it is, and if there is a need to have other apps independant from NextCloud 
Yes so far we have been using Passman on nextcloud. For my use, which is pretty basic, I am satisfied with it.
Only thing that I was missing is that you cannot share a vault, you can share your passwords though.
There is also this app: https://apps.nextcloud.com/apps/passwords but it seems less secure. It is service side encryption, not client side yet. Passman does client side encryption.
It think @bepolymathe used both. Maybe you can give us some feedback.
Yes, I did use both for a while. I confirm Passman is doing his job perfectly. It’s the only one I kept. It is safe and easy to use with Firefox or Chrome like (chromium, vivaldi, etc).
Personally, I think it’s a good option. If you use keepass, you can easily export and import into Passman.
I believe that Firefox also announces its secure password manager in the next versions (in version 63 if my memories are good).