I don't subscribe to hierarchy, but I'll switch primary for established relationship(s) and secondary for newer relationship in my answer since I feel that's what you're getting at.
If you’re kinky, have you found that kink levels decrease as you get closer/cuddly/more emotionally intimate? Or is it more of a “wax and wane” over the course of time?
Neither. For me it seems to take some time in a relationship to find the type of kink that works in the dynamic and with the specific person. Thus, so far I've found that my sex life with a partner includes more kink with time and the closer we get.
Is it often the case that when you meet new sexual partners you direct more extreme sexual behaviour towards them and away from the established relationship?
No way, haha!
Does NRE with others generally lead to less sex between established partners?
From what I've read here, it tends to vary a lot. Some people have a hard time focusing on anything but the new person, and in that case it is probably a risk if work isn't done to prevent that. For others (such as me), it's the other way around, and NRE for somebody also increases libido in general.
If you’re into BDSM play, do you have any restrictions on BDSM with others, or does everything go?
Anything the people involved feel like doing is fine.
Do you get jealous if your partner does something sexually with another partner you crave and hasn’t happened in a while? Do you talk about it, and how do you do this without them feeling you’re pressuring them into a certain sex act? Or do you just tell yourself it's none of your business because every relationship is different, and you’re glad he’s having fun?
I have limited experience with this, and in my situation it's been the case that if I haven't been able to get what I've craved, it has been due to circumstances (long-distance) and not differing wants. So, when I've felt jealous (occasionally), it's been helpful to remind myself that the circumstances would be same anyway, even if my partner wasn't having sex with somebody else.
I do think I might get jealous if I wanted to do something with my partner they were doing with another but not me. However, what I would do would depend on the reasons. If they know how important it's to me but don't feel like putting in the effort, that would tell me something quite negative about their priorities. But if they just don't feel like doing it with me (even after trying), then that is how it is, and I certainly can manage/get over the jealousy and be satisfied without that aspect [unless I can't in which case there's a problem regardless of my partner's sex life with others].