Many years ago I founded a project called ToroDB [1]. ToroDB had a vision very similar to that of FerretDB's: help MongoDB users feel at home on Postgres. This has far reaching implications, like allowing MongoDB applications to run without MongoDB (this is what FerretDB is essentially and what "ToroDB Server" was meant to be) or to replicate data from MongoDB to Postgres to improve the performance of analytical queries by several orders of magnitude (that was "ToroDB Stampede").
ToroDB ended up being discontinued. Timing was not right. At the time, NoSQL was exploding, and users "didn't want to look back to SQL" --until they learned the notable advantages, but it was a time consuming and hard job. Today, there's a much higher acceptance of SQL and most recognize that data querying in many cases goes through, or is significantly helped, by SQL.
I wish FerretDB a successful road and reach to where ToroDB didn't reach at the time. Good luck and congratulations on the 1.0 launch!
[1]: https://torodb.com