If you want to avoid @MainActor annotation you could use @unchecked Sendable annotation at class level to tell compiler you will be managing. Usually Net handlers are only one per app process so it will be safe.

In the line:

try await self.onNetworkStateChanged(networkConnectionState: state)

Try using an eternal reference for your net manager and avoid weak self

    private func setupNetworkMonitoring() async {

        let netManager = self
        await networkMonitor.startMonitoring { state in
            guard let self = self else { return }
            Task {
                // ⬇ ⬇ ⬇ Capture of 'self' with non-sendable type 'NetworkService' in a `@Sendable` closure
                try await netManager.onNetworkStateChanged(networkConnectionState: state)
            }
        }
    }