Are the writers of Discovery and Picard kicking themselves
Nope.
the correct use of the Riker/Troi cameo
The uses of Riker in "Et in Arcadia Ego, Part II" and "No Small Parts" were both the correct usages for their respective shows.
(and the ship they arrived in,
"No Small Parts" is set some time in 2380. "Et in Arcadia Ego, Part II" is set 19 years later, in 2399. It would be implausible for Riker to still be in command of the Titan after 20 years, particularly if he spent a significant part of that time raising a child with severe health issues as established on PIC. Not having the Titan in "Et in Arcadia Ego, Part II" is a completely reasonable creative choice.
and the music that accompanied it)
The different kinds of music in each episode reflect the differing creative goals of each episode. A story is not bad just because it was not the story you wanted to see.
And let's not forget what Iris Ira Steven Behr wanted to really do for the DS9 finale... Yeah I'm sure fans would have loved to know that DS9 was just the fevered dream of some 1950s African American science fiction writer.![]()
I agree that ending DS9 as Benny Russell's story would not have been the best creative choice, but I find your characterization of the Benny Russell character offensive. "Far Beyond the Stars" was a really important episode that tackled head-on themes and tensions that Star Trek ignores far too often, and Benny Russell deserves mores respect than to call him "some 1950s African American science fiction writer."