I think your story is great and reflects the ideal situation (besides the low libido hubby issue). Where we are completely free to have other relationships but we don't want to.
I'm just wondering whether you have talked with him about it. A low libido is often a direct result of our biological chemistry (in particular low testosterone). That is something that can be fixed naturally in my experience to great effect if he's just willing to make a couple of changes to his weekly routine. Have you looked into it?
You sound proactive and open so I doubt it has anything to do with the sexual chemistry side and you not igniting his fire.