[Quoth]

If you MUST use embedded fonts:
1) Only use FREE Fonts.
2) Subset so as to ditch all the huge amount not needed in some.
3) Test using KF7/mobi on an older kindle (only limited built in fonts)
4) Test on any Kindle that has "Publisher" in the Aa menu but without Publisher (the default).
5) Test on an Android or iOS epub app that ignores embedded fonts. Plenty of them ignore embedded fonts!

As long as it still "works" when the embedded fonts are ignored you're fine. If the ebook is done correctly the Headings will be Larger and maybe Bold. Styles such as italic, bold and bold italic in Sans, and also italic, bold and bold italic in Serif and italic, bold and bold italic in Monospace should work even on oldest Kindles and stupid apps. Test on a 4" to 5" Android 6 phone and a 10" Android 10 or later Tablet.

I do use embedded fonts, but not one book has a font issue on Amazon and they all look similar enough to the original even on dopy apps, default setting on newer Kindles and even on ancient mobi only Kindles. Monospace vs Sans vs Serif is the harder bit on oldest Kindles.