There is so much wrong with this I don't know where to even start. You want to "keep things simple" and not stand up a separate instance of MySQL/Postgres/Redis/MongoDB/whatever else. So, you:

1. Create your own in-memory database.

2. Make sure every transaction in this DB can be serialized and is simultaneously written to disk.

3. Use some orchestration platform to make all web servers aware of each other.

4. Synchronize transaction logs between your web servers (by implementing the Raft protocol) and update the in-memory DB.

5. Write some kind of conflict resolution algorithm, because there's no way to implement locking or enforce consistency/isolation in your DB.

6. Shard your web servers by tenant and write another load balancing layer to make sure that requests are getting to the server their data is on.

Simple indeed.