If you mean that your iMac has model ID iMac17,1 – that would make it an iMac (Retina 5K, 27-inch, Late 2015).
I could be mistaken, but I don't believe that DuetDisplay supports 5K resolution. Luna Display does, but if I'm not mistaken,
- The 2015 iMac would need to be running Big Sur or later (I believe you can upgrade it as far as Monterey)
- Refresh rate would be limited to 30 or 45 Hz even with a cable connection to help carry data
- Astropad themselves acknowledge that using Luna Display to drive a 5K display has some limitations
Astropad – Use Your iMac as a Monitor: A Step-by-Step Guide [2024]
"Like any product, Luna Display does have limitations and it’s not recommended for high-motion content. For example, I wouldn’t recommend using Luna Display for gaming or video editing.
Since Luna Display is squeezing a 5K display through a USB cable, when lots of motion occurs on-screen (like from video playback or moving a window quickly) the screen can temporarily become pixelated before it renders in at full resolution."