In digital any voltage more than half the supply voltage which is 2.5v(5v/2=2.5v) is logic high. Esp32 working on 3.3v has a logic high=3.3v. Unless in serial connection there is a 5v out on any pin of the GLCD that is the input for esp32 then level shifting is needed
Oled is good. Just take notice that with any type of display, check if they work in direct sunlight. If a display don't, tinted film can work.
Not correct.
There are no such general rules, you need to study the individual device datasheets for the actual levels.
Tried OLEDs, no way near bright enough for use in sunlight. My experience of tinted film is that it tends to make displays even dimmer.
Of course it makes it more dim 
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