The fact that RES was needed to even use Reddit is the real problem, hopefully nothing like it won't be needed here.

Yeah definitely, but if it's needed there's not too much choice. It would be an alternative to having submission OP's mark their own posts as NSFW though

Maybe allow tribes to mark themselves as NSFW and if your submission contains any nsfw tribes it's thumbnail will be hidden / marked nsfw? Instead of having a per-post nsfw system. This might cause some extra DB load unless it's indexed somehow.