Aside from the Shub-Niggurath worship (I'm more of an Azathoth person, myself), I agree with most things here. I'd just add to the list, group B I guess:

• aquatic animal husbandry and aquascaping (freshwater preferably, saltwater if you are really masochistic and have money to burn on corals and expensive equipment)
• model railroading

I feel these are more 'apex' hobbies, wherein you need a bit of everything (chemistry, electronics, an artistic sense, lots of patience) and they will occupy most of your time. You'd think electronics and aquaria are not the closest things, but just you wait until you feel the need to build an LED lamp with simulated day/night cycles and moonlight, controlled by an arduino.

The barrier to entry is fairly low - there are starter sets available and I've found that hobby shops of this sort are usually staffed by very knowledgeable people, eager to help newcomers. And, you can go as deep as you want and still have fun. You will also learn an absolute fuckton of things about what you choose to model with your hobby.

An honorable mention for homebrewing, which I don't even regard as a hobby at this point, but more of a necessity, like cooking.