It's not a Prequel Trilogy. That implies it's set before the Original Trilogy, when it's actually set in a completely separate universe except for X-Men: First Class. If anything, it's a Sequel Trilogy.
How can you rank The New Mutants as our favourite X-Men movie if it didn't happened yet... or Deadpool 3, for that matter.
Also, it kinda can be called a prequel trilogy, even though it technically isn't. X-Men: First Class serves, very well, as a prequel (and X-Men: Days of Future Past partially). X-Men: Apocalypse and Dark Phoenix eventually serve as a prequel to the revised 2023 scene from X-Men: Days of Future Past, so in that sense you can still call it that way.
Personally, I don't think one should separate Dark Phoenix from the prequel trilogy. It is solidly set as a tetralogy in that regard.