Yep, testing definitely has a hugely decreasing marginal value.
I personally like the approach of writing a small amount of "obvious" tests to detect large functional breakage, and add a test for a bug once it's been identified aka a regression test.
Imo it's maximum test value per effort. Of course it depends on how impactful a bug could be.
We get some bugs/issues that could be identified by better coverage, but most of our "bugs" stem from real world issues or differing views on functionality. Getting that right takes a ton of effort that simply isn't worth it right now.