Yes, the STX is a trim package for the XL. Kind of a half XLT deal.
Check the specs. The 2.7EB has the torque of the 5.0 V8 but at much lower RPM. It does have much less HP but still decent.
I simply don’t believe the 8500# GVWR you posted. All 2.7EB I’ve seen are well under 7000#. There is a 2.7 Payload Package that is higher than normal but not that high.
2.7EB WITH towing package (53A) is rated at 7600# towing. Bumper hole or Hitch Receiver (53B) is rated at 5000# towing.
If you think you might ever get a bigger TT, consider the 3.5EB with Max Tow (53C) (& preferable Heavy Duty Payload package). Once you pay for the 3.5EB, the Max Tow is a deal over the Towing Package. You get the 36 gallon tank, factory Trailer Brake Controller, 3.55 rear axle, reinforced bumper/receiver. IIRC, it’s like $400 for Max Tow over Towing Package. Otherwise the 36 tank, 3.55, & TBC are separate options. The HDPP adds a spring leaf, 3.73 strengthened rear axle.
I strongly recommend joining the Ford 150 Forum. They have a very informative Tow Hauling section. There is an actual “show us your Load Capacity” thread ;)
Buying off the lot, I had a choice between an STX with 2.7EB with Hitch Receiver or 5.0, Towing Package with TBC and 3.55 added. I bought the latter. I wanted those last two features and my wife was quite leery of turbos ;(
Note, the standard rear axle on the 5.0 & 3.5EB is 3.31. Apparently, the 10 speed transmission takes up the slack. AFAIK, 4.10 is only available on the Raptor, which has a crappy tow rating due to its suspension.