Screen Sizes fixed on itch!
If someone wanted to play GODDAMMIT BOB, I TOLD YOU TO WATCH THAT DAMN HYPER CANCER! but couldn't because of our botched screen size: It should be working now!
the main issue was us using the itch-native fullscreen button which meant I had to make sure the html itself had 1920x1080 specified in it - otherwise it cut off the edges and did all kinds of funky jazz. Disabling that and just leaving the unity template-button for fullscreen let me set the itch player size to a manageable 1024x576.

On 16:9 screens fullscreen should be fine, on other resolutions just stay in windowed :)
THANKS to everybody who played so far <3