That sounds like the forum software is ignoring the image DPI. When I screenshot in HiDPI macOS sets the image DPI to 144 DPI. When I screenshot in low-resolution 2560x1440 macOS uses 72 DPI.
I think it should be pretty easy to automate scaling any 144 DPI image to 72 DPI with Folder Actions.
That's what it is doing. The software is from the 1990s but nobody is going to change it now.
I can just shrink the image in the browser by 50% but I don't have to bother with a QHD monitor. I do wish that macOS would have a parameter to set the DPI of screen shots as current resolution. I run one of my 4k monitors at native resolution but 5k would make things too small. I can just use this solution for the time being. Of course iMacs of this vintage can die at any time but there's typically a stead stream of 2009 and 2010 models from $100 to $300.