W.r.t. the spoiler-y extras, you can always release them after that particular part in the releases was reached. So, if the extras refer to or spoiler chapter 35 and 37, you wait with the release of chapter 33.1 (the extras of the current volume) until after you’ve released chapter 37. That way the subjugates are included for future readers, but up-to-date readers won’t be spoiled.