> that at 100k DAU vertical
That's chump change size even for a medium EC2/RDS instance, which should be capable of tens of millions of queries a day without the CPU or disk complaining at you (unless all your queries are table scans or unindexed).
> my db doesn’t contain just “Bob ordered 5 widgets to Foo Lane” data
It doesn't matter, it's still just bytes. What will matter is your query pattern relative to the databases query planner efficacy, and how updates/deletes impact this.
> makes it very easy to manage data across realms
You can just as easily do this at first as separate databases/schemas on the same physical server, with different users and permissions to prevent cross-database/schema joins so that when you need to move them to different machines it's an easier process.
Everyone I know that has tested isolated multi-tenancy that wasn't dependent on legal needs ended up abandoning this approach and consolidating into as little hardware as possible. Heap Analytics had a blog post a few years ago about this, but I can't seem to find it.
Regardless, hope you success in your endeavor and that you come back in a few months to prove us all wrong.
If it's a game, transactions are usually either very few per player per day (login, begin or finish playing a level, say something in a chat, spend money on a loot box, etc.) or easily sharded (e.g. 30 commands per second for each of 10 players in a multiplayer level that lasts 10 minutes, not for each player all the time).