I would just tell her straight out that I'd like to cut down to once a week. If she asks about the reason, I think it being a lot of time allocated to therapy and effort is fine to say, that doesn't mean you think she's not doing a good job or the things you outlined in your post. You could throw in something about the office if you want to. I'm also wondering whether you'd have the option to switch therapists, since from your post I gather that you still think there's progress to be made, but maybe not with her, the office adding on top of that?

Ideally, a therapist works towards the goal of never seeing you again. To feel rejected by progress that your clients are making would be a red flag from a therapist for me. Or rather, if she lets you actually feel that and maybe tries to guilt you into not cutting back. A good T will - when they have the impression you're improving like she said about you - accept that part of this journey is you seeing them less and less. If they cannot deal with that, they need to work on that during their own time. I can see why if you already feel that it's more of a bit of a friendship than therapy, the worry is there that she might not be up to the task of not letting her feelings influence this part of therapy. But if it actually plays out like that and she tries to push you towards staying on two times a week, I think that would show you that you are indeed correct with your impressions of her and that cutting down or even switching therapists is really the right thing to do.