For DDDD, not gonna tell you how to watch but i've seen a lot of people going both ways. You can either watch episode 0 at the start or after episode 16. Episode 0 is chronologically out of order. I watched it at the start.
I just recently finished bingeing the show and really enjoyed it. Let me know how you liked it
From what i know the manga is in full chronological order. I don't know who's decision it was to have episode 0 at the start instead of at the end.
I honestly can't say i'm really on the fence as to where to watch it. I can see the merit as to watching it both ways.
Since i watched it first i had qustions where some were slowly answered and also some realizations about things being introduced that i already knew about because of Episode 0.
On the reddit for the series someone did a poll, yes is at the start no is after ep 16