So I tried making an origin that would have the phosphor shader applied to their screen when in direct sunlight, but uh... it still applies at night, throughout the entire night.

My guess is because sky light is still at 15 during the night.

I'm currently running on latest for 1.20.1

This has already been fixed in Origins 1.12.5 (for Minecraft 1.20.2). This was due to the ambient darkness of the world in the client only being calculated once; this also affected the brightness entity condition type and the light_level block condition type (if light_type is not specified.)

I won't be closing this issue since a backport for 1.20.1 is planned, but there are current circumstances that's blocking it, so it may take a while