Pain management, optometry, and many other non-psychiatric disciplines also rely on self-report. There's no bright line between the 'objective' and the 'postulation'.
That's just plain wrong on the optometry part. While there is a lot of self-reporting used in optometry because it's faster and easier, it's entirely possible to remove it, because you can't fake not being able to read stuff when your vision is too blurry. How do you think the DMV tests your vision? It's not self-reporting; they show you something that you don't know beforehand and you have to read it. Optometrists also use vision tests to gauge your vision as they try different lenses. If they really wanted to, they could move from static tests that you can memorize to dynamic ones to prevent "cheating", but why would anyone cheat on an optometry exam (like they really would for the DMV)?
Just being left in the exam room for 30 seconds unobserved would be enough.
No, you can't. Sure, you can memorize the optometrists' eye charts now, but that's because they're not expecting anyone to try to cheat, and just use some printed charts. Why would they expect someone with ill intentions? You're paying them to help you see optimally.
However, at the DMV, they absolutely can test you properly, because their machines (at least the last one I used a couple years ago) can change all the letters around however they want, and presumably every test is different (within the maximum number of combinations, which is surely too high to memorize). They're actually expecting people to try to cheat there, so they don't just use a single printed wall chart, they have a special device you stick your head in.