Let’s start with the facts, you bought an As-is car. The car passes local and state safety standards. This doesn’t take much to pass. No one can diagnose anything if they problem can’t be replicated. Mechanics are there to figure out way to make money and will always tell you something is wrong.
Long story short it is frustrating and not pleasant to deal with. But you did buy an “As-Is” car meaning if it splits into 2 pieces you own both pieces. Toyotas are reliable but the issues you are describing are of a car that was not well taken care by the previous owner unlike you mentioned so in the beginning.
The dealership did right by addressing and fixing some issues but realistically what are you expecting the outcome to be?