I use Google Tag Manager Server Side for receiving events from different event streams (Google Analytics for WEB, Google Analytics Measurement Protocol). I wonder if I can send events to my GTM Server with Firebase Analytics for Android, since I want to keep things neat and unified. The SDK itself doesn't seem to have any option to setup a custom transport url the same way as in Google Analytics for web. Any recommendations?
I suggest not doing it at the moment. To do it now, you would have to indeed switch from the FB SDK to using the measurement protocol, which is a lot more work for the developers.
Also, keep in mind that server-side GTM is still in active development. I followed up and it looks like they're planning to add clients for Firebase/Mobile SDKs, so it makes sense to wait till the new clients are available.