Yes, you can store the database file in a shared drive (not sure that's proper security though)

That is secure as long as you have a good master password

KeePassXC works really well with Yubikey too. I use a Yubikey and a short pin for the password.

You can use password protection on the drive as well as IP restrictions. You could even offer the drive only in your home network on your NAS.

You can use a combination of a password to together with a key stored locally on all devices

But how do you read the passwords from let's say iOS ? I don't see any iOS client on the website. Thanks.

You need to download an app. Search keepass on ios (I use android so I can't recommend one to you). They won't be made by the same people, but they can use the same password database

You use any Keepass compatible iOS client and access the same file (for example using a WebDAV share)

I setup two-way sync of the database with my android phone using syncthing - works flawlessly.

What's the difference between just opening the file directly on a shared drive? Possible conflicts with opening the database in multiple apps?

Wouldn't that require mounting of the shared partition on Android (or making it available is some day)? With syncthing, there is no such requirement.