After further testing, I reset Preferences -> Conversion -> Common Options -> Page Setup profiles back to Input Profile = Default Input Profile, and Output Profile = Kindle. And plan not to change them for specific conversions.
Reasoning: I want to preserve consistency of conversions within the library, and the books are readable on various devices regardless of page setup profiles, though image sizes may not be perfect for a specific device. In the future if I ever need specifically tailored output for a specific device, I could temporarily set Input Profile to whatever Output Profile was used in the prior conversion, and set Output Profile to the recommended profile for the intended device, then reconvert. But I doubt I'll bother.