Hi, I really, really appreciate your help.
The text size problem wasn't on the desktop, it was on my Galaxy A10 viewed in Firefox.
I just did a new export: Text Rendering: Images (preserves exact appearance), Mobile Device Viewport Zoom: Let in5 Choose, and Desktop Scaling: None and everything looks good. I could have sworn that was the first thing I tried this morning, but there was a lot going on and it's possible that I wasn't as focused as I should have been.
I did several tests this afternoon, and I can say for sure that if I export with desktop scaling set to "best fit," no matter which Mobile Device Viewport setting I choose, the text displays on my phone like the screenshot.
r7_test3 displays enlarged test like the screenshot on my phone:
http://www.areaviiclub.com/mntest/r7_test3/
r7_test8 (the one I just did) displays everything correctly on desktop and phone:
http://www.areaviiclub.com/mntest/r7_test8/
I think I've got it!