I'd really like to pursue a serious (possibly primary) relationship with him, and he says he loves me very deeply and probably would have married me if he had met me when I was single and mono, but he is mono, and he doesn't want my primary relationship to break.
That's all basically saying "I don't want to have a primaryship with you. I seek a marriage partner of my own. I like /I am willing to be your non-serious lover while I'm seeking my monoship marriage person."
Let me trying reframing what you wrote. Maybe it could help reading it in another way:
I am not sure what's best for me given the back story of how we become "FWB" type lovers and how we broke up. The experience taught me that I do not enjoy being in a casual FWB situation because leaves me emotionally confused. This experience has taught me I like being recognized as a "serious relationship" when I am someone's lover even if not sharing a primaryship together.
I'm also not sure what's best for me given the the fact that we have different wants: I want to be able to grow it toward a serious primaryship. He offers me FWB. He does not want more than that.
He wants to get back together like we were -- in a casual FWB lover situation.
- Should I accept this relationship offer? Be his FWB lover person when what I seek is primary polyshipping?
- Should I keep my distance and decline being his lover? Let the romantic feelings for him die down and keep it as friends only, "no benefits?"
So you have to evaluate now.
- Will time spent in dalliance with him cut into my dating time searching for what I really want -- a primary polyshipping partner?
- Does being a "casual FWB lover situation" feed me the way I need to be fed? What does it add to my life? What does it take away? Last time we were here I was left confused. How would this time be different? What expectations/behaviors of his would be changed? What expectations/behaviors of mind would be changed?
- Which choice best supports my long term well being in all my health buckets -- mental health, emotional health, physical health and spiritual health?
- Of those two options, which is the most self-respecting behavior that I can pick for myself? So I am trying to meet my own wants, needs, and limits?
If it were me? I'd let it go. Not shopping at the right store here. Be friends, skip being lovers. Because what in this list can you NOT get as his friend?
We have great conversations, comfortable affection, deep discussions, are one another's secret keepers. We spend a great deal of time together.
I think you can have all that as friends. Just not sex.
But you are you. You are the one who knows your own wants, needs, and limitations. So YOU are the one who has to choose.
Do you feel this "FWB lover relationship" that he is offering to you is worth the price of admission? And do you think you will you get enough return on your investment if you accept it?