Into the MCU?
I think Jeremy at The Quartering is correct. Dark Phoenix just bombed. You reboot with a cameo maybe best.
Or you can go this route.
1. You make a Prof.X and Magneto movie. Most of what made X-Men so great was their friendship/rivalry. Make their origins/history the center of the X-Men reboot just for the first movie or so. From there you can bring it alone.
At the same time give Wolverine a cameo in a Hulk movie before you do yours second X-Men movie after Prof.X/Magneto film. Finish the Hulk movie where Logan somehow meets Prof.X and invites him to the team hes trying to build.
That will give you 2 movies where the X-Men are involved. Both Logan and Prof.X/Magneto were the driving force to early X-Men and still are some of the most popular.
Then in the next movies you can bring the actually team into the fold. Prof.X runs into the Shadow King and that brings in Storm since they are around Prof. X at the same time. Shes not that popular but she adds depth and Shadow King is a good villian. You can also give Dr. Srange a cameo as Shadow King is well verse in the Astral world and all that you can also shoehorn Jean Gray into the mix. This also can add a new Dr Strange story arc for his movies. Maybe even Black Panther aswell, who knows.
There, you have Prof.X, Magneto, Jean Gray, Storm, Wolverine added in and you can also have minor introductions to Cyclops, Beast, etc in only 3 movies with 1 really being a Hulk movie. The cameos of Dr.Strange and Black Panther could help.
Not a bad start off point.