Actually a great idea.
Most apps I know use Firebase for tracking and Firebase is a great tracking tool. But in the world of privacy-aware tracking and it has some flaws (no details here today).
In a current project, I worked on a concept where we used server-side GTM as our major tracking endpoint for the app. All tracking event data is sent as Base64 strings to GTM. Here it gets decoded and transferred in “normal” event data.
In GTM the events can be passed on to whatever destination you like. In our case, we sent them to Amplitude. But you could also simply just write them to BigQuery.
For user identification, we do not rely on IDFA or similar. We just generate a uuid and persist it in the app. So if you reinstall the app, you get a new which is fine for us.
I really like this new setup. And a cool side-effect. App-debugging is nice with the GTM server debugging preview.