Since NIW is self sponsored, AC21 does not hold.
However, to reiterate what I said before, if your I-140 is not approved and you start working on EAD based on 485 filing, which is actually linked to your I-140,
if your I-140 gets denied, then you will be in trouble since all the days you worked on EAD are counted as out of status employment.
That is why it is never advisable to work on EAD until I-140 is approved.
In fact, even if I-140 is approved and one works on EAD, during the actual I-485 adjudication, they can always ask to re-verify that you are still working in the same field as your NIW/I-140 petition. But that's a different issue.
Mine is also a EB2-NIW case. I'm working on EAD now, 485 still pending. But I started working on EAD only after I-140 was approved and I'm still working in exactly the same field as my NIW.
However, note that if you do not have any other option, i.e., you cannot get H1-B, and you can't extend J1, then you have no choice but to use EAD. You have to take that risk. As far as USCIS is concerned (and all applicants who are still waiting for their GC/485 approval), it does not matter at all whether you are in this situation intentionally or non-intentionally.
Many people are in different kinds of situations, mostly for no special fault of theirs.
It is solely your problem and you have to learn to deal with it.
My 2 cents.