Actually crafting jet should be something like doctor profession 4.
It says on the wiki it's a separate profession, so it should be your 4th.
Requirements should be a bit high, perhaps even higher than doc level 3.
Perhaps it should require that you have doctor level 3 as well to make it hard to acquire this profession so the wasteland isn't flooded with jet.
The crafting system for doc is indeed not that nice.
With exception of buffout all the the interesting stuff is at level 3.
You have a long cooldown for picking up 9 flowers or 9 roots (7.5 minutes).
Yet crafting super stims only gives you a 1 minute cooldown (never crafted normal stims, so I assume it's the same).
As for profit, doctor profession is 1 of the most profitable ones for high level stuff.
stimpack - 1 root (free), 1 flower (free), 1 bio med gel (20 caps), 1 hypodermic (1 or 2 caps in shops, 9 with Jules in New Reno) - total 29 caps max, sell for 155
buffout - 10 meat jerky (not sure if Grisham charges for it, 5 caps in shops), 1 chemical component (free) - sell for 275
radaway - 1 chemical component, 1 bio med gel - total 20 caps, sell for 300
super stimpack - 2 roots, 2 flowers, 1 bio med gel, 1 hypodermic - total 29 caps, sell for 310
mentat - 1 chemical component, 7 fruit (free) - sell for 400
pyscho - 1 super stimpack (29 caps when crafted), 1 bio med gel, 1 chemical component - total 49, sell for 700
rad-x - 1 chemical component, 3 bio med gel - total 60 caps, sell for 450
Cooldown seriously limits your profit though.
It becomes really profitable if you're in a faction wih an outpost and the faction supports your doctor alt with gathering.
Do a chemical component run with a few people, pick fruit from the outpost, have them gather roots and flowers when hunting, etc.
A tent near VC is very useful.
I disagree on the losing combat skill.
Why lose something you already have?
It would mean people simply level up to 21 as fast as possible and than buy all 3 professions at once.
Although I'm sure a lot of people with doctor skill buy all 3 professions at once already as 1 and 2 aren't that useful.