First off, for your purposes (as stated above), virtually any phone will work for the phone part. You don't need an iPhone 6 or 6s.
Second... Phones aren't optimal for lots of data entry (i.e. writing in forums). Even reading forums can be a pain because of the smaller screens (IMHO). An iPad is much better for that. An iPad is also much better for watching videos.
However... An iPad is not as portable (they're not really pocketable, even the mini's). It's not socially correct to haul out your iPad at a restaurant and hack away at it. However, it's not polite to use your phone and ignore people around you either... It's just less obvious.
You could look at an iPad mini. More portable than the Air's, more functional for some of your wish list items, cheaper than an iPhone. But then you're not going to be making many phone calls on it... Which is kind of the purpose of getting an iPhone. You don't even mention calling or texting in your wish list.
C