Would Venus be a scorching hell if its day wasn’t 255 days long or does being 20% closer to the sun make that much of a difference?

It is its wicked green-house gas atmosphere that is responsible for most of it.

Given Venus’s slow rotation, if it was just rotation then the side away from the Sun would cool down a lot. But in fact both sides remain about the same temp. This implies the atmosphere quickly distributes the heat, so the rotation rate is pretty much not a factor. It could have an 8 hour day and stay the same.

It’s greenhouse effect (“so-called”, yadda yadda) plus closeness.

Okay, I was figuring that Venus’ day was 255 days long before the atmosphere became so dense (assuming Venus was more like Earth billions of years ago) was somewhat earthlike and when Venus began heating up and released the carbon dioxide from carbonaceous rocks which broke down from heat.
I hope that made some sense.

Here are some numbers. The equilibrium blackbody temperatures are calculated for planets without atmosphere at the appropriate distance from the sun.

       blackody    Observed
     temperature  temperature
        K    (F)        K
Venus 	325	(126 )      700
Earth		277	(39)       250-300

Being closer to the sun is only a small factor in the hellish temperatures on Venus.