Jason Isaacs is still my guy. Like Avery Brooks, his is the voice that I hear when I read Commander Blanx's dialogue. Despite being 57, he's still in pretty good shape, as evidenced by his turn on Star Trek: Discovery a few seasons ago. That said, it's been just under seven years since that initial casting post, and if I want to hold out again until 2030, I should probably recast with an eye toward longevity.
The best thing for me to do, if I'm not going with a literal representation of the comic book imagery for the villain, is to think outside that box. Cast backwards both from the actors that I've already selected and by finding a character for performers I'd like to see in any production of this sort. For instance, I enjoyed watching the full series of The Good Place over the last year or so. Chidi Anagonye was my favorite character on a show that plays heavily with themes of philosophy and existentialism. Chidi is the moral center of the show, thoughtful and concerned to the point of near total inaction. William Jackson Harper was excellent in that role, but he was also a rare point of interest in the mostly interminable Midsommar, playing a duplicitous toxic academic douchebro. It wasn't a complete departure from Chidi, as both characters are academics focused on observation, but it did demonstrate range and Harper's ability to make me turn on him while playing an unsympathetic character.
Parker also has elven, slight otherworldly features, including a severely arched brow-line and intensely puckish smile. In a better world, he could have been a perfect Captain Marvel/Shazam with those Howard Porter eyebrows of his. When he smiles, he radiates joy, goodness and warmth. Unfortunately, we're on Earth-Zachary Levi, one of the really crappy Earths, and so I'm instead casting for subversion. What if you took the expectations that come from this guy's presence and prior roles, and employ those qualities for the charismatic demagogue Commander Blanx? Though both men are in great shape, Jonathan Majors has nearly half a foot on Harper, but Blanx was never played as a major physical threat. His whole thing was that he was a charming fascist who bullies and connives his way into a position to commit global genocide for his sole personal gain. I think Harper could act the hell out of that, and I'd love to see his malevolent glee played against J'Onn's quiet virtue.