I can't provide too much feedback, as I've read a fair amount about the app signing service but I haven't used it. There's a few advantages I can think of:
- You can use Android App Bundles
- Your application is protected against the loss of your keystore
- If your keystore is compromised your app is protected from a 3rd party creating a modified build
Android App Bundles is another kinda large topic; it's basically an alternative way to publish your application. The play store uses the bundle to produce multiple APKs based on the requirements of different phones ( screen size and CPU architecture ), so that it can remove anything that isn't required by that device. As the play store has to sign those APKs you have to use the app signing service for this. Unfortunately construct apps don't really benefit from this, so there's not much to be gained from it.
Sounds like your already fairly protected against #2.
#3 is fairly important. Signing an application is basically your stamp as a developer to say that you made it. Several services rely on the application signature to ensure that the app is legit. If your keystore is compromised somebody could potentially produce a modified version of your application, with your signature on it. This would mean those services would trust the application. The amount of damage that could be done with this obviously varies on the service, but it's definitely something you want to avoid. Provided you keep the keystore in a safe location, and with a very strong password on it, this likely won't a problem. But by using the app signing service the malicious actor has to have both compromised your keystore, and have access to your application on the Google Play Console. If the keystore is compromised then you can revoke it with Google Play, which isn't something you can do without app signing.
As a final note, your probably aware of this but once you publish an app your stuck with your decision!