I think this idea is great (for images/videos). However it should display that the spoiler/nsfw spoiler was added by moderators rather than the message auther. Bluesky has a similar system for marking NSFW content and i think their UX could be applied to Fluxer. However i think this should only apply to images/videos since imo text would be too easy to mess with.
Thread
Comment by @SteveLinkNoah