tough that J wont be okay with you being poly. that seems like the biggest issue from my perspective. You cant ever count on anyone changing for you, and you cant change anyone. So unfortunately it simply could boil down to either choosing him or choosing poly. or maybe you choose him for a while and know you are going to be poly after him. so many people are "poly" over time....e.g. they get their multiple relationships simply by having one after another but never two at the same time.
so you were with z and he would be mono with you but now you are okay being poly, and now j is there and he doesnt want u to be poly. i hate how hard it is for people to even hear about poly. the concept is just so unknown by most people.
when i met my wife i was up front with her about being poly, but the truth of it was that i wanted her more than i wanted a poly life. so i married her, but she knew my mind and respected my ideas and agreed with them in theory at least. that made me feel comfortable enough to marry her.
i couldnt imagine marrying her if she completely disagreed with my ideas. so good luck with making your decision with J. it's coming up for you again.
you could try an experiment with him. something like, "how about we try out just one or two months (you decide what you propose) of being together with me being poly, and lets see how it goes. then lets sit down and talk about everything and go from there. i dont think it will be as bad as you imagine, because im going to work very hard to show you how honest and considerate i can be about it. then when we talk at the end if it just isnt working, then revert to where you stand now with him.