It should’ve been Achebe with some high power villain like Moses Magnum to keep things visually interesting. If they wanted to bring in Atlantis for the end of the trilogy just have Achebe steal the Serpent Crown and let Moses Magnum cause an earthquake that either destroys part of Atlantis or brings their civilization closer to the surface. That would’ve set up the final movie.
Or they could’ve had Namor be a background villain in BP2, supporting Achebe to destabilize Wakandan vibranium distributed across the globe to soften the surface world up for an invasion. Essentially taking advantage of T’Challa’s new policies and forcing him to be that darker more Priest-like figure from the comics. I don’t inherently have an issue with Namor in a BP movie, though I’d prefer a more in-house trilogy that uses original BP rogues and ties them into the fabric of the MCU (and by proxy the comics). If we can’t get that there should be a more ceremonious buildup to the introduction of one of Marvel’s oldest characters. Tbh he should be the villain/anti-hero of Fantastic Four film where he’d probably get used much better.