And really it sounds like Android might be the way to go for you. The App issue sort of faded into the background in this thread. If it is a pressing issue, then Windows Phone 8.1 and Windows 10 Mobile will not be an immediate fix.
The Moto X is a fine looking machine. I still like the Nexus 6 better for screen size and resolution, faster LTE potential, Chipset, RAM, camera has Optical Image Stabilization, stereo speakers, wireless charging and battery capacity, more guaranteed and broader support... But that's me.
http://www.gsmarena.com/compare.php3?idPhone1=6604&idPhone2=6649
Again the Moto X should be a very nice plug and play upgrade from your 8x.
Think about this. When has paying over time EVER been cheaper than immediate purchase? Your phone is no different. If you can afford to buy a phone outright, do so and get them to explain how much that saves you each month from your bill...