I gave it a try, to be honest, I feel it too cumbersone, I expected it to use already available mutations not just copying existing types.

What I really want/need is custom logic that run IN the resolvers of already available query/mutations with the request as input and the possibility to either return an error or modify the payload before it hit the database

but I know is still in preview, will check it again when released

Agree. This is one of the issue we have with Hasura and it's what made us switch to Postgraphile.