Is that possible? According to Esri's documentation for DataStore:
Your ArcGIS Enterprise portal's hosting server must use the relational data store provided through ArcGIS Data Store.
In any case, both DataStore and RDS are a "hands-off" approach to database maintenance. A lot of the day-to-day stuff is managed internally, like vacuuming and updating. That can be nice, especially if you're not an db admin.
RDS gives you more direct access to the database for creating tables and views and the like, but you may be a bit hamstrung by RDS's own settings, as you cannot install or update DB extensions in RDS. We had to migrate our Enterprise GDB to its own EC2 instance because of version incompatibility between RDS and the latest updates of Enterprise/Server. Definitely a deal-breaker.
It's also worth mentioning: RDS is more expensive to operate than a comparable EC2 instance. And while you can schedule an RDS / EC2 instance to turn on and off at intervals to save on expenses, if you're using it as your portal's DataStore / Hosting Server, it will need to be on at all times, otherwise your Portal won't function properly. So a difference in cost is going to matter all the more.