One nuance to "Why trade Pettersson and not Miller" has to do with trade protection.
Pettersson doesn't have any trade protection in his contract until July 1, 2025. A player cannot have a NMC or NTC until they reach the season in which they would be old enough to qualify as a UFA. This means Vancouver could trade Pettersson anywhere they want up until July 1, 2025. After that date, the player gains complete control over which team they would accept a trade to, significantly diminishing Vancouver's leverage in trade talks. This is the same scenario Montreal faced with P.K. Subban—they ultimately traded him before his NMC kicked in.
Miller, on the other hand, is already in his extension and has a full NMC until July 1, 2028. This means he can already determine which teams he would or wouldn’t accept a trade to. As a result, Vancouver’s return in a potential trade could be limited if Miller decided he would only waive his clause for a specific team. This is similar to what happened with Ryan Kesler, who forced Vancouver into trading him to Anaheim for a relatively weak return.
So they could trade Pettersson now and get a relatively rich return—or they could trade JT Miller and get a relatively limited return.