Asynchronously fetching data from the database
Your async/await usage doesn't look fine because the chain it's incomplete. FindList should be async and named FindListAsync and it should await the result from QueryAsync.

There are also a few other things that botter me:


since BaseQueryDataStoreAsync is an abstract class then QueryAsync should probably be protected and not public - otherwise someone can use it for any query
the CategoryQueryService should be injected
the GetHandler doesn't look very useful
making the dbConnectionString a const is a terrible idea - you could/should use appSettings.json to store it and IConfiguration or IOptions to get it
setting QueryAsync's default parameters to null where they are null anyway (source) is pretty pointless - instead just use the overload with a single parameter.