GMike,
Welcome aboard, there are many here with vast experience, I am not one of them. My path is somewhat similar, in that I am also a married man in my 50's mostly mono for most of my married years. From everything I have read, it is much easier to start poly rather than open an existing marriage. But like you said, you only just heard of poly. It wasn't on the menu when I put in my order 23 years ago.
I know nothing of your marriage, but here are a couple of things which I think give me a fighting chance of being able to have my marriage move to an ethically non-mono marriage.
1) My wife had an affair 10 years ago it was very short lived, but it put me through the, ringer. It essentially transitioned me from mono to poly, because I lost all attachment to my marriage, yet I also completely 100% forgave her. The affair doesn't even exist in my mind. I don't believe even in the term affair. Couple this with my wife's excessive drinking, I essentially built a wonderful life for myself with or without my wife. So I personally went through the process of letting my attachment to my marriage go. Mostly out of necessity.
2) After the affair my wife and I played around with swinging. There were a few times when my wife was out of town and one of our "sister wives" kept me company with my wife's approval. This experience greatly reduced my wife's anxiety around sex outside our marriage.
3) I have had a very close relationship with another woman for the last 6 years, it has been strictly platonic. Only recently has my wife sobered up (literally) and realized the extent of our connection. This has been very threatening to her, but this threat is dissipating.
So in essence, I already believe that I am poly, as I have love for two women. I tried to be as ethical as possible in this secondary relationship. I realize that having this relationship somewhat private was not ethical, but when your wife explodes at you every so often in a drunken rage at 3am, you tend to hold your cards a little closer. In addition, as my wife has sobered up, I have made it very clear that I love her and want to continue on with our marriage. Why the hell would I have waited all of these years for her to get her shit together if I didn't love her and value the marriage. However, I have also stated that on principle, I will not throw this other relationship under the bus, so to speak, and poly is essentially who I am at this point. So going forward, how I express my polyness can be very negotiable, but I will not cut off communication with a dear friend to alleviate my wife's insecurities. So bottom line is I am willing to loose my marriage out of principle, but as this unfolds it is unlikely to happen. But I had to risk it.
What would be more likely to happen is my marriage would fall apart due to my wife choosing to drink and not be spiritually and emotionally present to work through our issues. This would also be a deal breaker. So I am working on my marriage on two fronts. The sober front is actually more important at this point. But it does not negate the poly goal.
So I have three questions for you.
1) Are you ready for your wife to fall in love with another man and would you be ok with that. Are you ready for her to fall in love with another man and leave you for him because she stays mono.
2) Do you think your wife has the ability to deal with the emotional trauma of this type of transition (on your wife's part). For me it was brutal, and for most people who have spent a lifetime being indoctrinated into the mono culture, I believe it is also equally difficult.
3) And three are you willing to leave your wife to be true to who you are.
And a fourth question.
4) Do you have the patience, compassion and love for your wife to nurture her through this difficult time.
and as long as we are asking....
5) Do you have the internal fortitude to press forward with your own beliefs even when it appears insurmountable and terribly difficult.
So in summary, it is not very easy, but I do wish you the best.
CON: I have waited and tried asking again later over the years and each time I bring it up it upsets her more
PRO: I love her, she is my best friend and I don’t want to leave her.
I hope the tone of my response is not too blunt or negative, but I was reading between the lines of your post to assume that you were up against a stubourn and inflexible wall. The one thing that you may have going in your favor, is that you love your wife. If she can see that your desire for poly is not a negative reflection of your love for her, then you have a start. But there is often such a big divide between an intellectual understanding of poly and it's implementation.
My recommendation is to take the very long and slow approach to this process. Lots of reading, and lots of soul searching. Could you get your wife to read the "Ethical Slut" with you?????? That might be a start.
Best of luck to you. I am sure more here will chime in with more seasoned experience.