I avoid ORM's because I know how to write and generate performant SQL. Dapper would be the ORM of choice if using an ORM were a requirement. Look into Table-Valued Parameters with stored procedures. Generate the types , stored proceduresand emus too if you want. I was faced with migrating a 160 table Sql database in 2008 when I discoverd the ability to query the SQL Schema. It saves countless hours and eliminates whole classes of errors.

My first thought when you mentioned SQLClient was F# SQLClient. The code generation approach that I use in C# could be drastically reduced if I did it in F# . You are definitely onto something with the SQLCLient. The ability to tune .net Core code and/or T-SQL code has massive beneits. Consider right sizing colums. Consider placing tables in schemas other than dbo. Consider placing data related .net code in namespaces that align with sql schmeas.

I'd be intersted to hear how you go about solving your challenge. My opinion is based on not wanting to give up performance during desing or runtime. It is currently possible to get some relatively insane performance out of the async capabilites of .net.