I'd add to this to say that the only reason the 24" iMac even still fits in the lineup is because Apple doesn't sell a standalone 24" display.
Not sure that I agree. Apple insists that the iMac is in their DNA. It's also the case that if you're recommending a low-end Mac desktop, it's often easier for a consumer that doesn't want to then go shopping for a display, keyboard, speakers, mouse, and webcam to just buy an iMac that comes with all of those things. The 24-inch iMac is a return to the 1998-2001 basic model of "hey, here's a complete computer for the every person; just plug it into power and turn it on and away you go!". This was something that customers of the lower-end iMacs needed and higher-end-use-case customers of the 27-inch iMac never did.
as a fellow 2017 5k iMac owner, so when is it the time to make the transition? My system works well enough, but wish I didn't hear those fans when pushed and not excited that Sonoma is not supported (even though there's not much new to it). I have been eyeing a Studio Display and Mac mini, but it feels disappointing to essentially end up with an identical screen. How much is it really a performance boost for non-content creators?
The computing prowess has nothing to do with the size or specs of the display. I'm not too sure where the disappointment factor comes from.
The fact of the matter is that the 5K display in your iMac is trapped in that iMac and can never be used with any other machine without a serious hack job taking place.
A Studio Display will work with any compatible Mac until Apple either stops supporting it or until it dies. You will likely replace whatever Mac mini/Studio/Pro that you buy AT LEAST once over in the time that you own that display.
As for the speed difference between a 2017 27-inch iMac and even a standard M1 Mac mini is night and day. And that's especially true if you have either a hard drive or a Fusion Drive in said iMac.