Ah ha! I think this is the key. You are worried that the lack of isolation implies lack of intent. But that is absolutely not the case!
The lack of a Sendable conformance is a very clear statement to the compiler. But I think it might not feel that way right now because you do not have any checks turned on. So the only warnings you will ever see are being surfaced by the explicit MainActor annotations you add.
You are second-guessing the compiler's ability to guarantee safety for good reason! Without warnings, there is no safety.