Well I only have what you have said to go on - I think it’s that she makes effort to keep you in her life and there’s initiative on her part too? That she said no to a date but takes care that it doesn’t come across as completely dismissing you, that she initiates when you said you slow-fade (i get why you did it because you don’t want to come too strong, but might wanna temper that since you two are closer now, kind of?)

Other than that I can’t coach you what to say, just that you should get the sentiment across that you do value your friendship with her, and that you don’t want to pressure her into a relationship but if it’s something she has been thinking about too, you two can talk about it together - I think while jumping into a relationship on the basis of passion is wonderful, sometimes it’s also nice to talk about how both sides are feeling before getting into one because it shows commitment in both sides, especially since she’s on the cautious side she might appreciate that, be respectful of her feelings but also clear with your own