You can do pretty well with latency in some regions. We are in the same building complex with AWS/GCP/Azure sometimes, and latency between Fly apps and the database should be sub millisecond.
If you can find good matching regions, you can setup a wireguard peer for connecting your app to the DB.
Our goal in life is to solve managed postgres in the next few months. Either us managing, or with someone like Crunchy Data running on top of Fly. So “hold tight” might also be good advice.