At: https://www.renderosity.com/settings/

When I set "Primary OS" to "Macintosh", it resets to "Linux" (which is the default for some reason, despite being the least common). Same thing happens if I try to set it to "Windows".

Also, that OS hasn't been called "Macintosh" since the mid 1990s (Macintosh System 7 was the last OS named that). It's "macOS" now (and was before that "MacOS X", "OS X", and "MacOS" at various points between "Macintosh" and "macOS").

Next, when I click "Update options" in that section, the resulting message tells me it updated the Social section instead (though it does actually mostly update the "Customization options" section, aside from the "Primary OS" bug).

And "Birthday" under the "Public profile" section seems to have an off-by-one error. It shows a date that is one day earlier than what I entered.

Also, the "Software" tagging under "Customization options" has redundant duplicate entries for Blender, Daz [Studio], Photoshop, and Paint Shop Pro. It's also missing a lot of obvious stuff, most glaringly GIMP. And it lists "Autodesk" as if it's a piece of software, when that's a company that makes a whole boatload of relevant software, just like Adobe does. That includes Maya, which you list separately, while not listing Red, AutoCAD, and various other Autodesk 3D modeling tools. I would also add NIFSkope as worth a tag. And, "3DS MAX" should be read "3DS Max"; the "Max" part is not an acronym (they just have an all-caps logo, like Daz, which is often mis-written "DAZ" as if it's an acronym). Oh, and while you list Substance Painter, you're missing Substance Alchemist and Substance Designer. Given that people tend to use them together, you could maybe do a combined tag of "Substance Alchemist/Designer/Painter".

I believe DAZ is an acronym actually, I think it stands for 'Digital Art Zone'

Yes. DAZ is short for Digital Art Zone.