Are there any small keyboards that are good for playing? I'm afraid that taking out a 74-key device will be enough of a nuisance every time that it would discourage me from keeping on learning. How about a 40-key keyboard that's fine for fingers?
If you seriously want to learn and play piano, even as a hobby, that is the entry level.
I also disagree with using any of the popular apps as anything more than supplemental tools, but there are some very good adult lesson books can definitely form the backbone of your learning. As long as you're aware that posture and technique is a struggle for self-taught pianists, you can look for videos and make some conscious effort to improve at it.
Buying an $80 smaller midi keyboard is a good way to get a cursory feel for things without spending the full amount.