i'm shooting educational videos for youtube
with my canon 70d and 50mm 1.8 stm lens in good light conditions
when i upload the video on youtube it looks good on Full hd but display it on 360p
the quality became very bad and blurry
setting : /60 2.8f iso 160
here an exemple :
and here is a person who creates same type of video that i shoot but the quality didn't change a lot when you see it in 360p on youtube :
Not sure where you're from and if the Internet over there is steam powered, but 1080 / 720 isn't that bandwidth intensive anymore, especially with this kind of content where there is very little motion.
I don't think you can do anything else but having your talent use wider felt-tips / whiteboard markers, and write a big bigger on the board.
Of course upload a high quality version, and do note that not all compression software is the same, eg if you use old Quicktime to make a H264, it may be 5 times bigger than a better quality version from modern encoders.
(And, it does not look that bad over here, so I would not worry about it.)