Oh hey I'm doing this too! I finished BBS a couple of days ago, just ordered 2.8 so I can play DDD and Fragmentary Passage before 3.
The games definitely get better after 1, by the mid to end of 2 Sora is an absolute machine and the ridiculous combos you can pull of are really fun. Ditto with BBS, once you meld some high level magic that game is hilariously broken but still fun. I actually played and really enjoyed CoM, even though the general consensus seems to be to just watch the cutscenes online.
I didn't bother with the side story movies, just watched some 15 min overview on YouTube and I got everything I needed from those.