Hi,
From what I understand, Gygax began planning AD&D in mid-1976. The Monster Manual came out in 1977, perhaps because it was the least disruptive to apply to ongoing campaigns at the time (some of which had been going for several years).
The modules were released to a crowd that had already been playing (O)D&D for several years and were looking for new challenges to make things harder for veterans. Consider that each of the modules D1-3/G1-3 is a tournament (competitive play) module, and S1 is Gygax’s hardest dungeon by his own design. So it stands to reason that “Advanced” at the time implied “So you think you’re good, and you’ve seen and survived everything? Try these.”
That shifted once the Players Handbook was disseminated in 1978-1979. The message there seemed to be “Hey, reroll, we’ve fixed the game’s problems and you don’t need to house-rule everything anymore.” This makes more sense from an historical perspective if you read The Strategic Review issues and the early Dragon issues (1976-1978) in order.
The “hey, reroll” movement was ushered in with T1 in 1979. Granted the giant frogs are a TPK to most non-veteran players, but hey, there wasn’t a scientific method to these things yet. =P The DMG has a story of its own; basically it was really hard to write, needed the PHB foundation, and there were printer nightmares, so it was 1979 with some people not having it until early 1980.
1980 marks the year when Gygax got swamped with D&D explosive growth following the Egbert incident, and thus others were writing for the system. Note that A1, C1, C2 and S3 are tournament works. The level spreads began to change because you had the “hey, reroll” crowd plus the veterans and a fair distribution of players across all levels.
Q1 is a different case; Gygax was hoping to write it but had too many irons in the fire, so it got written by Sutherland, if I recall.
In 1981, A2, A3 and A4 were wrap-ups of the A series by different authors and their individual takes on competitive D&D. There are some interesting design notes in Dragon. I1 is a separate deal, pet project. L1 is Lakofka bringing his campaign to the table (hence low level), while U1 is the first TSR module contribution from the guys across the pond. They had already been doing their thing pretty extensively with White Dwarf and Games Workshop. Fiend Folio was around this time as well.
TLDR, these were the forerunner products. 1977, first RPG hardcover. 1978, first massively revised edition of an RPG (I think). 1978, experiment with pre-planned dungeons following Palace of the Vampire Queen’s success. Etc.