Wow, Beth, that is quite a unique situation that you have with the guest who has been staying Monday-Friday for 6 months. I agree that it would be overkill to ask the guest to leave a review each week. If I were in your situation, I wouldn't ask such a guest to leave a review each week either.
However, on the bright side, the main advantage to being a Superhost is that it makes your listing more attractive to potential guests, but you already have a regular guest Monday-Friday every week. In other words, in your situation, being a Superhost is only helping you book guests on Saturdays and Sundays. I have a feeling in your situation the difference between being a Superhost or not is probably negligible.
If your regular guest ever stops staying so consistently, then once again you will have an opportunity to get new guests on a regular basis who can leave reviews for you, and hopefully regain Superhost status quickly. Either way, I think you'll be in good shape. Hope things continue to go so well for you!