The problem with Orbot's VPN mode is it doesn't support chaining with another (real) VPN, since Android only natively permits one VPN at a time. This is a problem for users in places where the Tor network is blocked (and Tor Bridges / obfuscated relays are, too, or are unreliable), because you need an initial hop outside of the Great Firewall in order to reliably link to Tor.
Amethyst app works with Orbot (VPN mode). Posting this from an Android phone using Tor.
Overall I agree privacy could be improved a lot at protocol level and in different nostr clients. I had written a post about nostr privacy recently that youihjt find interesting: https://consentonchain.github.io/blog/posts/nostr-privacy/
Discussion
In contrast, if the app builds-in tor support (or its own tor service), it'll work regardless of how you're connected or your network is layered (as long as it isn't breaking TCP?).
Another problem I faced with Orbot is that if you want to use it in vpn mode but with only selected apps you cannot enable the "always on" mode, otherwise the others apps don't work, but with this setup if Orbot crashs/disconnects the selected apps are free to use the main network and so leak.
Waiting for multi-vpn support (perhaps is GrapheneOS working on this?) built-in Tor support in an open source app fixes this.