What is considered a good / great LP/PH value?

MTF is a system measurement; it is not a lens measurement. So, what is "good" depends on the capabilities of the system as a whole and the combination of limitations. E.g. lack of lens sharpness due to spherical aberrations, image softening due to AA filter, and max sensor resolution.

I'm assuming a typo and you are referring to the DX D7500. That has a vertical resolution of 3712px, max 1856 lp/ph, 116 lp/mm. Of course, that is theoretical, based solely on sensor pixels, and would be at a very low contrast. But, it is not even theoretically possible for your camera to reach 2400 lp/ph; so applying that standard makes no sense. Also note that you may see Imatest results in l/ph... given that measure the D7500 could theoretically reach 3712 l/ph. And people mix up their units of measure all the time (or they are not clearly stated).

The very best lenses reach over 80 lp/mm on a very high resolution sensor w/o AA filter (e.g. Nikkor Z 85/1.8 on a Z7). But, in general, 30 lp/mm is considered a good level of decency. Why 30?

What 30 lp/mm means is ~ 1.4 MP recorded on a DX sensor... that probably sounds crazy low; but the CoC standard for image sharpness is .02mm for a 1.5 DX sensor. Which equates to 50 l/mm or 25 lp/mm (the accepted standard requires less than 1MP recorded on any size sensor). And 30 lp/mm exceeds the 25 lp/mm standard.
But that 1.4 MP result is also an MTF50 number, and there is almost certainly a lot more resolution at lower levels of contrast which can be improved (sharpening is just adding contrast). And it's not like the typical human's vision just fails below 50% contrast.

Nikkor Z 85/1.8 Z7 test in lp/mm:
https://www.lenstip.com/579.4-Lens_review-Nikon_Nikkor_Z_85_mm_f_1.8_S_Image_resolution.html
Nikkor Z 85/1.8 Z7 test in l/ph:
https://photographylife.com/reviews/nikon-z-85mm-f1-8-s/2