I also find it increasingly difficult to evaluate the models I actually do use. Sometimes each new release seems identical or only marginally better than the previous version, but when I then go back two or three version, I suddenly find that oder model to be dramatically worse. But was that older model always that quality, or am I now being served a different model under the same version name?
It's all just so opaque.
Regarding evaluation, I've found using tools like promptfoo (and in some cases custom tools built on top of that) are useful. These help when evaluating new models/versions and when modifying the system prompt to guide the model. Especially if you can define visualizations and assertions to accurately test what you are trying to achieve.
This can be difficult for tasks like summarization, code generation, or creative writing that don't have clear answers. Though having some basic evaluation metrics and test cases can still be useful, and being able to easily do side-by-side comparisons by hand.