For what it's worth I don't know that it's the right time to be speaking of opening your marriage. It sounds to me like you are still doing a lot of work on yourself and your sex drive. The biggest thing that stood out for me was the 2 miscarriages. I think your brain may have linked sex to pregnancy to pregnancy loss. See what happens once you get the IUD in and sex can just be recreational again.
Bringing in somone else to a relationship tends to magnify issues that you're having in your relationship. Poly is a lot of work, it will bring up everything that has ever been a concern for you. Since you have already identified your lack of sex drive as an issue and are working on it, I would wait until either you have your sex drive back or you aren't feeling insecure about your lack of sex drive before having the serious conversation about opening your marriage.
Everything will be ok in the end. If it's not ok it's not the end.