Here's an update!
I got the first batch of test boards back. The flex cables look good, and seem to work fine, both in terms of direct soldering, or when used with a connector socket. They are 0.26 mm thick, which was the closest to 0.3 mm (the recommended thickness for the sockets) that I could reasonably make them. I didn't want to use stiffeners on either end since I wanted them to be able to be soldered directly as well.
The breakout boards have some mistakes. The component numbers (U1, C1, C2, etc.) got messed up on most of the boards when I panelized them, but that's an easy fix. The big issue is that I got the SD socket backwards. Since I haven't used these very narrow sockets before, I apparently didn't assume the correct orientation. Whoops. I've re-positioned and re-routed this component. It's a bit tight, but I still managed to stick to >6 mil traces/spacing. New revision below:
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