Moving to distributing self-signed APKs

Sorry, I missed in the Google Play Console UI that you can indeed “Add key” to an existing package. Then the steps are indeed as described on the ticket you linked:

1. Add the F-Droid key’s signature (d9423d31b482bc1638bf553b6ac1a5fdf07e85ac197de6f0d796b1a0f8912273)
2. Publish a new version to F-Droid that includes your adi-registration.properties
3. Upload the resulting APK to complete verification

To be clear that would seem to remove the need to move to self-signed APKs.