Welcome to the forum
I would ensure your primary relationship is in fact healthy and in good standing. Building into a poly relationship, adding pieces to your existing relationship is like jenga. One wrong piece on a poorly build foundation could crash the entire thing down. If you believe your foundation is solid, then enjoy the ride.
Have you ever investigated D/s lifestyle as a sexual choice for you and your partner. You might find it sparks another part of your conversation about sex as well. You might find, with how passive your husband is, you could take on a more dominant role with him. Finding another person for yourself to be dominant with you.
It sounds like you are on the right track. Good communication, you are aware of your challenges and have a great understanding of what you want. The only way to make sure he is on board, is to keep communicating