Wanting to make a shed into a sheshed/office. Sooooo my question is how should I insulate it? There is no rafter vent. Is spray foam insulation sufficient for Brazos valley weather (humidity). Or should a rafter vent be installed?

Fewer consequences w/Humidity using Closed cell foam &/or Rockwool.

Office would need at least a window/thru-wall AC or one w/heat aka PTAC, some require 220v.
This avenue would not vent to outside humidity.
I would require an office window.

LG window ACs have served us well.
Sizing experience w/LG 11.3 SEER
Min. windowed rooms use 450sf/ton
40-50% windowed rooms use 200sf/ton (perhaps more SF if shaded 100% or North wall)

I don't think you need a rafter vent unless you had some kind of attic space?

I'm sure someone smarter than me will weigh in.

No need to vent your conditioned space. Biggest issue with insulating a shed is working with the limited 2x4 stud bays. Spray foam will give you the most R value per inch, but most people don't want to spend that. Stick with typical fiberglass or stone wool insulation and just know that you'll need a decent HVAC.