Plethon lived during 14th and 15th century before the fall of Byzantine empire and if you want to credit one guy it should be Averroes (Ibnu Rush) who lived in the 12th century, at least two centuries before Plethon. He was highly regarded by the western scholar as the next Philosopher after Aristotle but not so much in the muslim world. He definitely has access to the Greek works including by Plato and Aristotle. I would say his many translations, commentaries and original works of more than 100 books were instrumentals in re-awakening the western European interest in Greek thinkers, an area of study that widely has been abandoned after the fall of original Roman Empire (not Byzantine) [1]. These do not include the numerous Greek, Chinese and Indian translations, and further contributions of Abbasid empire's scholars (Averroes belongs to the Muslim Spain empire) that includes the well-known House of Wisdoms but later was burnt down in the 13th CE by the Mongols. It's said that the river in Baghdad turn into blue due to the inks of the disposed books. Some of the books managed to escape the utter destructions from copying and circulations in the Muslim Spain and other part of the Abbasid empire that really helped the Renaissance movement later on.
https://en.wikipedia.org/wiki/Encyclopedia_of_the_Brethren_o...
But, yeah, I’ll stand up for old Marsilio’s influence any day. The story is amazing.
Regarding the origin of the Renaissance era please check my other comments on the potential root of the Italian Renaissance movement.