Yes! For a bi guy who wants a safe supporting relationship to make some of his fantasies come to life, your comfort zone should be a bi guy's dream zone. It's not a straightjacket by any means.
NOTHING in your limits is unreasonable. And nothing says you have to enforce those limits IF things go well within them and IF you feel like changing them. You're only offering him what you believe you can deliver, which is openness without separateness.
He's not your fuck buddy, he's your fiancé, and your openness to share certain fantasies (and not just put up with them, but be enthusiastic about something you think is hot) tells me you've definitely got enough of a brain in your head that this might just work. Any Bi guy with a brain, who also was interested in something somewhat open (not all are) should be amazed that he was lucky enough to find you.
Secondly, if it is good sex, it should not "quench his thirst," it should make all of you want more of it more often. Or at least have fond memories of it that make you horny every time you think about it. You might each want different things out of a fling like this, but if it is worth doing, it is worth enjoying. And if it is worth enjoying, it is worth repeating. If this is supposed to be "one last cock" before he "settles down" then you're all probably heading for disappointment. If you're willing to be open (within limits) now, you should count on being open (within limits) 5 years from now...once in a while, as horniness and opportunity allow, within the limits you both agree on, in your open, trusting, up-front very cool marriage.
Thirdly another guy is not just a sex robot. You could just get a blow-up doll or a
dildo for that. This is another real live human being, and all of you are allowed to make emotional connections with each other.
Basically it's not only allowed; it just happens. Accept that, but also accept it does not mean that all the emotions are in competition, or that it is a zero sum game with winners and losers. Your fiancé is going to have a physical and emotional connection to someone else, and you are too. It's not the same emotion, not the same physical union, but it is there.
And if that sounds like a future you both want, and you both still see a path forward together, with optional "visitors" in the short or long term, then you have the makings of a happy life together.