I don't see the anti-competitive part there. There are good reasons around consolidating resident daemons and only using a single TCP connection with a special keep-alive configuration.
And from experience, it's perfectly possible to not use FCM, at the cost of a persistent notification that alerts the user about this, so they can get rid of abusive apps.
Maybe not exactly anti-competitive (but maybe): It requires you to have hundreds of megabytes of Google blobs with root permissions on your device, which some people don't like.
If course this pales in contrast to Apples actions.
I think you can but you still have to link to Google play services, which is not possible for open source apps in f-droid since they need to be able to run on de-googled android which lacks Google services.
Aren't microg supposed to get installed first on the device? Which mean if the user doesn't have it, then your app won't run? Or can you embed the FCM support into the app itself so it doesn't depend on google play service or microg availability?
They may not perform anticompetitive behaviour like bundling (as they have already been fined for in the EU over Android and Play Services).