Perhaps the answer is to consolidate business logic in the database with constraints. This also protects data from manipulation outside of an app, like with scripts or ETL.
Putting more logic in the db often results in terse error messages though, so the app needs to deal with that and make them nicer for the end user. I wrote about one possible approach here - https://begriffs.com/posts/2017-10-21-sql-domain-integrity.h...
> weak version control/deploy solutions
There are solutions that allow you to store migrations in version control, apply them to different database targets, and run consistency checks. For instance http://sqitch.org/ but maybe you're familiar with it already and regard it as one of the weak tools. Thought I'd point it out though.
> a new programming language to the stack
plpgsql is somewhat gnarly, but it is well adapted for the database. You know what it's going to do, compared with managing mappings from some other language. I never tried LINQ though so maybe the mapping would be more pleasant than I'm imagining.