What I can suggest is to implement ActivityPub directly instead of using APIs.
The Fediverse (see: https:/en.wikipedia.org/wiki/Fediverse) is a large federated network comprising of different software and protocols. The main protocol with the largest support currently is ActivityPub, thanks to it being a W3C recommendation. There are also the web- W3C recommendations (webmention, websub, etc.). There are also other protocols like diaspora*, DFRN, OStatus (actually succeeded by ActivityPub), Zot, and others.
From what I see currently, the Fediverse is being approached as “Mastodon” and “Pleroma”. I strongly suggest to approach it as ActivityPub and you’ll find it easier to connect to the fediverse directly than using a particular software, especially since Mastodon and Pleroma are only two of many software which are connected to ActivityPub protocol/network – there is Misskey and Hubzilla to mention two more.
Also, there is no need to create accounts one for Mastodon, another for Pleroma, another here and another there. By implementing ActivityPub directly into Fdroid, “identities” (or accounts if you will), simply has to connect to a particular App to receive updates/notifications.
email@example.com adds/follows/subscribes to @firstname.lastname@example.org
email@example.com adds/follows/subscribes to @firstname.lastname@example.org
email@example.com adds/follows/subscribes to @firstname.lastname@example.org
email@example.com adds/follows/subscribes to @firstname.lastname@example.org
When App1 uploads a new update, the fdroid system will create a short update. Anyone who is following @email@example.com will receive this update.
There is no need for the maintainers and/or devs to create an announcement account because their App’s fdroid repository link is an actual ActivityPub “channel”.
You can also free your main fdroid account from posting updates of different apps that for sure most followers won’t care about. By implementing ActivityPub directly instead of relying on creating an account in software1, software2, software3, you are also giving users the option and an easy way to subscribe to only those apps they do care about. While your main fdroid account will serve as an announcement for the project itself, not an “app update” announcement “channel”.
For me, this is the best option and less headache. You don’t have to worry about what-if a dev transfers to another server, especially since there is no cross-software account transfer.
Take a look at projects like PixelFed, Write.as, and PeerTube. PixelFed is an Instagram-type software federated with the ActivityPub protocol. Write.as is a blogging software. While PeerTube is, you guessed it, a YouTube-type software which is also federated in the fediverse.
When a user in a PeerTube server uploads a video, anyone who are following that channel will receive an update. If someone writes a new blog post in a Write.as server, same thing. Likewise when someone uploads a new photo in a PixelFed account in a server of their choice.
For Fdroid, those channels are the Apps itself.
Here’s the kicker. You can implement a comment system through ActivityPub that will show as a “feedback” or something when the App is viewed (maybe as a separate tab). The App maintainer or developer can also be given an option to reply to those if you want to add that as a feature. It’s all up to you. It doesn’t have to be two-way if it isn’t feasible.
If you can, implement ActivityPub directly, it’s the best option hands-down.