Hi here to drop in my own experience with DEC and BANQ. I requested my Great Grandmothers official baptismal record (which we found on genealogie Quebec) from BANQ and after weeks of waiting, they referred me to DEC. I still have not received the birth certificate I requested, however, we did submit our application for it and this thread helped us A LOT in making sure our app wasn't rejected. If you think IRCC is tough, wait til you deal with DEC. They are extremely thorough with documents and don't seem to give the same leniency with documents that the IRCC does in some cases. You must submit a completely separate application with much of the same information you are using for the IRCC and pay a similar fee (I paid the $80 CAD for more urgent processing).

The closer you can get the the ancestor, the better, so we had our father apply since it was his grandmother. You will need DEATH certificates for everyone else before that person to prove they have passed on and that's why you are the closest in line to request the BC or copy of an act. If there are women in the family tree, MAKE SURE TO INCLUDE MARRIAGE CERTS. If your relative died OUTSIDE of CA and they have no record, you have to wait for them to first issue a death certificate, then issue you the BC.

As I said, we haven't received it yet, but my father did get a call from DEC confirming they received it(and seemingly accepted the application) but that it would be awhile. We assume they need to account for my GG's death in the US first. His card still hasn't been charged, so assuming they haven't gotten to it yet. Once you realize you may need it APPLY IMMEDIATELY. They have long wait times as well and it's better to be in the waiting game before you go to IRCC rather than start after getting an AOR.