You got it all wrong, there are 3 timelines:

The original X-Men movies:

X-Men: First Class

X-Men

X2: X-Men United

X-Men: The Last Stand

The Wolverine

Logan

The youngster's timeline:

X-Men: First Class

X-Men: Days of Future Past

X-Men: Apocalypse

Dark Phoenix

These two have X-Men: First Class in common because it all begins there, but after days of future past they create a new timeline that branches off into apocalypse and dark phoenix.

The Deadpool movies have their multiversal timeline, they are in another part, another earth of the multiverse, but of course, there are variants of the X-Men.

The new mutants can be consider to happen on its own earth as it has no relation whatsoever to any other of these titles.