Remote access Mac Mini M2 from imac performance

For serious photographic work, you really need a display that is directly connected to the computer to get the true results.

When using via a remote display on a computer with Apple Screen Sharing or say Microsoft Remote Desktop, the images would be transmitted as highly compressed JPEGs which would not give a faithful representation of the processing results.

That would improve the processor speed for Lightroom, but cause an Image problem.

If the 24" M2 iMac is out of the running?

The M2 Mac mini and a decent aftermarket 27" display would be a decent upgrade option.

Another depending on how much you use Lightroom?

Is to keep the 2017 iMac for awhile longer and live with the speed or switch to using macOS Photos.

One other thing in terms of your iMac and Lightroom performance. How much RAM does your iMac have? A bare minimum for Lightroom or Photoshop is 16 GB (forget any Adobe minimum claims) and really need more as they are massive memory hogs. Before departing from the Adobe "sphere", I was running my iMac with 32GB to get the best performance.

The 2017 is not a monitor it is a computer. If Lightroom is running too slow on the 2017 iMac, then why not just use the native Photos app. for editing instead of Lightroom.