The latest Aquaman & The Lost Kingdom test screenings are happening this month. The latest cut that was screened on June 8th reportedly had over half an hour of Amber featured.
I think Dolphin's great too. Just not as a love interest for Arthur.
I think the easiest route they could go would just to swap out Mera for Dolphin or some other love interest. Honestly, they would probably love the opportunity to mix in diversity or whatever.
But it would all depend on whether they're even going to make a 3rd movie anyway... DC seems to be changing course every other month, so we'll see.
It's been years since I read PAD's Aquaman, and nothing about Dolphin stands out in my memory. I didn't mind her at the time, but I was missing Mera -- who seemed to be another victim of DC's desire to erase or irrevocably damage girlfriends/love interests of its Silver Age heroes. Peter David admitted online that he was getting ready to bring Mera back into the series when it got cancelled.
I wonder whatever happened to the first artist of that series -- Marty Engeland, I think. He was very talented and then disappeared.