I’d love to have my own password manager that syncs between all my devices without relying on a third party server, but I don’t know of one. Do you?

A physical notebook is so easily lost I wouldn’t even consider it.

I use KeepassXC and Syncthing. It's not a single password manager doing both the managing and syncing, but it works very well, and I sync between phone/laptop/desktop without any third parties.

Do they work on iOS?

Keepass database is an openly documented format, and there are several apps available on iOS that use it and can store the database in your iCloud (ie syncthing is not required, unless you need to sync to non-iOS devices)

Just search for keepass in the app store (Keepassium, KeePass Touch, Strongbox, ...)

I see there are a lot of Keepass apps in the App Store. How do I trust the developers not to steal my credentials though?

I don't know about iOS but if it saves it to your Files and lets iOS deal with the synchronisation then it should not have any network permissions?

(I use KeePassDX from F-droid it does not have network permissions)

Great name.

Frank Dux was a fraud! :)