I think Giffen did a better job building up Thanos than Starlin.
The point that he took over that mini is really what gave birth to the version of the Marvel cosmic universe that we have today.
I realize that you love Annihilation, and I share that sentiment, but your quoted post above is just straight-up hogshit. Thanos was almost an afterthought in the Annihilation stuff Giffen handled. He was one of many threats, was never as big a deal as Annihilus, and really didn't do much until he got killed.
The stuff Starlin did with Thanos in the 70s (the classic Warlock/Avengers/Mar-Vell/etc stuff) and again the late 80s/early 90s (Thanos Quest, Infinity Gauntlet, etc) just pisses all over Annihilation in terms of establishing Thanos's threat level