Does Sarah want to change? Is she working towards feeling safer with your relationship with her husband?
If she is working towards changing, honestly you sound really impatience. This is very different then swinging and it takes time to wrap you head around it. She might need things to help her out. It sounds a little like you are unwilling to help her out if it means less time with her husband.
Do the three of you agree on what you are working towards? What does she _want_ the relationship to look like? What do you want it to look like? What does her husband want it to look like? Are you on the same page?
If she doesn't ever want you to have a private sexual relationship with her husband and isn't working towards it... then yeah, this might not be the right place for you. If she does want that but isn't there yet emotionally I think you could make it a lot easier on her by slowing down, being 100% honest and being patient. She needs your help, your love, your support. Not you standing there tapping your foot and saying "you aren't going fast enough for me".
If you are all on the same page you have time. I know it can feel like you don't. It can feel like everything you want is being kept from you and that is unfair but this woman is opening up her heart, her marriage, her life, her home and that is scary.
If she doesn't want you to ever have the relationship you feel like yo want then you need to figure that out. If you all want the same thing then you need to relax and try to find the best ways to support her while she figures things out and trust that she would do the same for you.
Poly is hard. It is complicated emotionally even for people who can't live any other way. We don't have a lot of examples to look at and society doesn't exactly come running to support us... that is why it is so important that we support the people in our relationships. It can be frustrating and slow but if you are all working towards the same thing you'll get there. It just takes time.