That choice is completely up to you and I can't tell you whether you should or shouldn't. I just want to say that I'm in the same position and I think some other people around here may be as well. When my relationship ends, my plan begins. I can't stand the idea of him being the one to find me and wondering if he could have helped me, so I'm making the decision to wait. But I have a feeling I'll push him away naturally like I do with most other people, so in my mind it's just waiting for the inevitable.
Only you can make the choice of when to do it and whether or not you two should be broken up before, it's a really sensitive issue and I couldn't advise on something I haven't fully figured out yet. The reality is, though, that even if you're broken up when you ctb, the news will still wreck him the same way it's going to wreck everyone who cares about you. Even if you end on bad terms, it will hurt. I've known people who have mourned lovers they weren't with for years when they've passed. But it will, obviously, hurt more for him to lose you while you are together unless he understands your position. Ultimately, I don't know if it's immoral or not. That would depend on who you ask. In general, it sucks to live just for other people's sakes so the waiting feels immoral for me, as I do it, like other people are holding my life hostage. Maybe it's one of those things that's a little wrong on both sides.
I think we're just going to have to be okay with hurting others if we're going to commit to our plans to ctb, and I'm trying my best to make peace with that as well. It's inevitable that those around us will feel pain and we should be understanding to that, too. I wish you luck with everything and I hope that you can find a solution that works for you and makes you feel the most at peace in the end.