But my general personal opinion is that vets can tend to over investigate.
When you are getting into a series of tests for a cat (or for yourself), some good questions to ask are along the lines of:
what will change depending on the outcome of this test?
what would be the worst case scenario of not doing this test?
what are the risks of running this test?
Sometimes questions like this have led the vet to explain there is basically nothing to be done regardless of findings and the reason for the test is “to know”. If they suspect a diagnosis for which the treatment is $20k, and you will under no circumstance be able or willing to pay $20k, then there is effectively no treatment. So the real purpose of the test is to satisfy intellectual curiosity. If the test itself is expensive and/or uncomfortable for the animal, why bother?
If a likely outcome is more testing, then you need to keep digging. Endless investigations with no likely change in treatment is a service for the human to feel like they are doing the right thing, not a service for the cat to be more comfortable or healthy.
As an example, a cat developed a limp with no apparent discomfort for no known reason. He was an adventurous outdoor cat so some kind of injury was possible. I took the cat to the vet who found nothing on physical exam and xray. The vet wanted to then do a bunch of other imaging. When I learned what he thought could be going on, I read about the conditions. The treatment for those of which treatments even exist, is a very expensive (equivalent to several months of my take home income), invasive surgery with long recovery time. Complications always possible with such a procedure. You then have to do long term physical therapy to the cat. I am sure the cat in question would be miserable about all of this. And after all that, the chances that it will actually solve the problem are less than 50%. So therefore we elected to stop investigating.