Actually, Data has been in Starfleet for 22 years, prior to his 1st command in Redemption pt 2. He enrolled in the academy a year before Beverly Crusher, according to the basic profiles shown in Conundrum. So he's already been in starfleet for like 18 years when we first see him, and right from the start of the show, he has come to acquire the rank of Lt. Cmdr, & holds 3rd in command status on board the flagship of the fleet
You can't get to those levels whilst still being a raw automaton. The point of the character was that major development has already taken place, just like all the characters
The development the OP suggests very likely did take place..... already. Now we are onto this next stage, where it's more of an esteem issue. Due to his lack of a solid reference base, he is unwilling or unable to recognize his own, albeit limited, emotional awareness
He just doesn't have an emotional state comparable to humans, and since he was made by one, to resemble one, and lives amongst them, he & others wrongfully presume he has no emotional awareness, which I see as having never been the case, even if characters professed otherwise, & despite the fact that he eventually obtains more complex programming, which gives him fully developed human emotions.
One of my favorites moments is in Data's Day when he states that if he had emotions, he would be rather nervous about going to the neutral zone, & then notices his own fingers nervously tapping. That's a moment where even Data notices his own physical manifestations of apprehension. The show is loaded with those kinds of moments. I can't say if it was always the planned intention, but it was the overall intention nonetheless
Ira Graves had it spot on. Data is the Tin Man, always had a heart. Yes, he is the absolute opposite of the Spock character. Spock is a being of overwhelming emotion, beyond that which humans possess, and has learned to suppress it into a near absence of emotion, whilst everyone around him never truly believes he is emotionless. Contrarily, Data is a being who has such an understated emotional awareness compared to humans, that everyone, including himself see & profess it as a near absence of emotion, all the while, he seeks to attain the emotional level intrinsic to humanity, never noticing his own intrinsic emotional awareness
The only way the premise the OP suggests would work is if Data wasn't even a Starfleet officer on the show. Only then could the story of an automaton developing into a fully realized being be told, and how is that going to regularly tie into the voyages of the U.S.S. Enterprise? Not very easily imho