Tough one.
Your first two requirements rule out a "traditional" tablet like a Cintiq. They are big, heavy, cumbersome and need to be tethered. Make no mistake the Wacoms are absolutely best of breed and even the newer knock offs from the Asian market are quite capable if budget is an issue but they are really not practical for portable use.
Tablets work well as sketch drawing devices and falling down the Apple rabbit hole should not be a major deal breaker. I know plenty of people who are Windows based everywhere else but use iPads because they are the best choice for their specific need. Get what works for YOU is the take away here.
Autodesk has some nice apps for iOS and Android.
Drawing App & Painting Software | SketchBook | Autodesk
There is a free feature limited version you can get to try out on your current Android tablet / Smart phone.
But tablets don't necessarily provide the same pressure sensitivity as a Wacom digitizer does, storage and RAM are also limited.
Your "best of both worlds" option would be a convertible laptop like the Lenovo Yoga's or Fujitsu Lifebook.
You get all the benefit of a portable device with reasonable horsepower and somewhat of an upgrade path and a true Wacom digitizer.
You can often find older models in the $500ish range.