export animated gif not collapsing redundant pixels across frames

An animation I did, starts with 16 frames of the exact same image.  Then, 10 frames of a fade to 50% dark.  Then 10 more frames of no change, etc.  I did it this way, because, apparently, an Adobe Animate GIF animation can have only one frame rate -- even though it's possible to set varying frame rates in an animated GIF.

When I export this as an Animated GIF the file size is rather large.  In my search for ways to reduce this file size I discovered that all those frames where nothing changes are NOT being optimized.  In other words, the Export utility is not collapsing these frames, to conserve file space [like another GIF Animation tool I occasionally use].  How did I determine this?  I tried chopping out, all but one frame, in each of the places in the animation, where nothing changes, and the files size shrank by a factor of 6!!

Am I missing some optimization setting, or is this just a limitation of the software?

Another thing I can do in this other GIF Animation program I own is: Set individual frame rates for frames in the animation.  There doesn't appear to be a way to do this in Adobe Animate.

Why don't I just use my other GIF Animator?  Because doing so would involve another tedious step: I would have to save the GIF from Animator, then load it into my other GIF Animator, and then delete the superfluous frames, then set a slower frame rate on the remaining frames.  It would be so much nicer of Animate did this.