Replicating the thin, soft crust of Portuguese Sweet Bread
I'm the OP. After a lot of experimentation and reading, I've found that the solution is to use steam. A lot of articles talk about how steam is necessary for a hard crust so that made me think it was the opposite of what I needed.
I learned that steam does several things, including keeping the top surface soft for longer so the bread can expand further, preventing the crust from burning, and helping the starches become a gel that turns into a brittle and glossy shell. But in almost all cases, recipes call for steam to be used only during the first portion of the bake, and then removed to allow a hard crust to be formed in the latter part after the bread has finished expanding.
I did several experiments where I added steam for a little bit longer each time, finally just using steam for the entire bake. That's when I finally got the ultra-soft, wrinkly crust like in Portuguese Sweet Bread. After going back to read the article mentioned above, I realized that its suggestion of adding a tray of boiling water to the bottom of the oven at the beginning of baking was what I needed, but to also keep the water in the oven for the entire bake.