Is it possible that some of your wife's hesitance is that she doesn't want a second wife in the house?
It's one thing for you to say "I am poly and I want to share my love with another person too" and even if your wife got to a point where she could fully support that, it doesn't mean she necessarily wants to share her home with someone else, and that's completely her prerogative.
In the same way that the girlfriend is an person with needs and feelings, your wife is too, and if she wants to keep her living arrangements the way they are, then you may need to rethink your plans.
Ultimately, I find it unrealistic to have an attitude of "anyone I date is with the intent to marry" whether as an addition to a poly relationship or as a monogamous person looking for the love of their life. The moment I stopped looking for a life partner, he fell right into my lap. Literally, it was within a few months of me deciding to stop "having relationships" and just start "dating" people and living for myself. Then *plop* the universe says "ok, now that you GET IT, here's your soul mate"