I’ve been thinking of the latest book of each TrekLit series as series finale, given how it’s increasingly unlikely to get new novels, or at least new novels in the same continuity. Except for VOY: To Lose the Earth and PRM book 3.
While not intended as finales, the last books of TNG, DS9, ENT and PRM give us a feel of saying goodbye while the voyage continues. DTI actually concludes their story, with room for sequels. Both ENT and S31 finished major plot lines, and DS9 Gamma was DRGIII’s foray into a new storyline - the continuing voyages again, so to speak.
It’s sad to see the characters go but it becomes easier the more time passes. Plus some concepts survive in STO (Kai Kira, eg).
We’ll see if the new Kurtzman projects will happen and how they rewrite the worlds we love, to bring us new adventures.