Woof. That sounds very complicated. If you need that kind of write concurrency, use an unlogged table in postgres [0]. Then you don't have to invent a whole sharded thing yourself.

[0] https://www.postgresql.org/docs/current/sql-createtable.html...

There are so many unfortunate footguns with unlogged tables, that I'd argue that the goroutine route is preferable.