, I have a slightly different question for you. What is the deal with your LTR? Why are you together so long but not married? Is that him or you? What do you want your life to look like 6 years from now?
I don't recall, but assume you live together if your LTR is like a marriage. Are you really meant for each other? I just wonder if one reason you've put so much time into this "friendship" is because your LTR guy doesn't meet some of your needs (companionship, conversation, admiration?) but you don't want to "cheat."
I lived with my H for years before we finally married and in hindsight it was a huge mistake, because when you live with someone you can't date different people to find the "right" person, but the years are ticking away with no real commitment toward building a life together. It's like you are a couple, you get along fine, but if you're not quite right for each other, you have no opportunity to explore that.
It seems like if FWB was really the guy for you, you would have figured that out a long time ago when he was single and gotten out of your current LTR. Now so much time has gone by, you've bonded with him and maybe a small part of you finds him exciting compared to your LTR, but neither of these two guys ever swept you off your feet.
Just food for thought.