Since delegate is set to be on main queue, previous call to DispatchQueue.global (apart from using global) wasn’t correct as well, and it called main actor-isolated method anyway. To address heavy operations it would need to set delegate to be not on main queue in the first place, which might involve much larger changes to the code.
It is kinda OK with custom actor executors, as you now can create a separate actor and set it to specific queue, offloading main thread and keeping Swift Concurrency checks in place, except that you’ll need to handle delegate conformance with opt-outs, but I consider this minor to the case. So at least this part is actually pretty OK. There are other APIs in AVFoundation specifically that I’ve found harder to use with Swift 6 checks.