The biggest change for me was when I switched to strictly typed languages. It doesn't matter if it's Go or Typescript or whatever. As long as it has types it dramatically improves maintainability and ease of scale.
Introducing types to the server side language has never done anything for me other than making sure that the types are in sync with those of the database while creating a lot of tedious overhead converting data that the application itself might not ever need to process to a particular type.
The Elixir gradual typing approach has always seemed like the ideal balance to me.
But in general, moving from PHP, Python and JS (where I started out years ago) to languages like Java, Flow-typed JS, and even Scala at work have made me realize how much dynamically-typed large projects loose in maintainability vis-a-vis statically-typed projects. Yes, excellent test coverage can help mitigate this, but static types go so much farther when used effectively.
I wish it were an easier thing to do, but I think as far as maintainability and codebase size scalability go, it's hard to quantify properly without metrics that are usually only tracked in enterprises.
I'm trying to think about how to make this more clear in the original post.