Given that I submitted my request over a weekend and I'm getting close to the deadline for applying, I decided to read some other threads about the switch to StockPerks. Based on what I'd read in other Cruise Critic forums, it appeared there is a way to avoid the brokerage account link and manually uploaded docs similar to the old way. It seems that this manually uploaded file approach has been successful for others.
So, I decided to check out the app to assess my comfort level. I did not want to have my late booking and request prevent me from receiving the Shareholder benefit.
While I, too, do not appreciate the use of a third party app and would never agree to use the "connect your brokerage account via a third party" approach, I was able to submit my redacted brokerage account pages (same as I sent to Carnival) manually in lieu of using the additional third party connection service (it's called Plaid). My understanding is that once my Shareholder status is confirmed, the process for requesting the OBC for each cruise is just a few additional clicks. I'll post my results when I get them.
FWIW, allowing the manually uploaded file approach instead of the brokerage connection is likely a nod to those of us who don't trust this new electronic method. Also, I suspect it means on each subsequent cruise, it'll require that I go through the manual upload again since they can't re-verify my stock ownership electronically. But, I'd be fine with that.