I have two methods that work perfectly fine for this purpose. I have used both for my apps that are available on Google Play store and so far there are no issues with Google.
The first one involves using ZipSigner mobile app. Here is the tutorial.
It is a quick method and can be reused over and over again. Also you can save multiple keys with different names to sign different apps.
Second method is a solution for PC, the advantage of this is that you don’t need to send the aab file to your phone and back to your PC again. You need to download aab-signer here then follow the instructions in the video tutorial below. Actually it is so easy and once you get it right, it will be the fastest method you can use.
With the above methods, you can develop AAB files of any size and put as many assets as you wish, you are only limited by the Google Playstore file limit.
Cheers as you build your app, all the best! 