Actually, that would be the standard expectation for something like this -- if you're asked to come back for a guest spot on a show you were once a regular on, or reprise that character in a spin-off, you would typically make one full episode fee at your series regular rate. If it's going to be TRULY in and out, you can probably get away with offering them 25K for one day, but that really is the floor for this kind of situation. Though I grant this is a little unusual, in that so much time has passed -- usually there haven't been 18 years since the last series regular appearance.
The fact that they so extremely lowballed him is the first bit of news I've heard about Discovery that has me giving them the side-eye. It's insulting, and most producers and casting directors would have known better.
You can always draw upon personal connections, which I'm sure is why DeForest Kelley agreed to do the TNG pilot for scale, because Gene was involved. But no one on Discovery worked with Michael Dorn in the TNG/DS9 days, so there would be none of those favors to call in.