I mean, I'm on the opposite end of the spectrum: I'm 6'2" and ride a 58, and have been curious about even hopping up (or at least test riding) a 60 cm to see how that fits. So idk if I can give you best suggestions for tiny frames with non-sloping top tubes, as I haven't really been keeping an eye out for em.
What's so bad about the sloping top tube? With frames so tiny, you're kind of limiting yourself from otherwise good bikes...