I thought both Thanos and Drax were terrible, and don't matter all that much for understanding Annihilation in general. I almost didn't read Annihilation because I thought they were so bad. They were some of the first comics I read once getting into the medium, however, so that may have something to do with how I felt. They're not particularly noobie friendly.
Read the first few issues and it's very exposition heavy, not liking the writing style in general. I'll finish it now that I've started though, just to get more info on Thanos as a character. I read the Infinity Gauntlet years ago, but don't remember much. Wasn't particularly impressed with that series either.