I'm not saying I'm right, just sharing what I find works for me personally. I make the desktop 1280 wide and use Scale To Fit Width in the Export Options. My mobile sites I keep at the default 480.
Text size for Normal Text I use 18 or 20. 16 is acceptable, but is a tad small on an 8" tablet. Text size can look differently for different fonts; I use Fira or Monsterrat most of the time for no other reason than they both have a wide range of variants and I like 'em.
Having more than 2 variants is just adding more things to go wrong. Not a big enough to scrap them and start again though, unless you'd enjoy doing so.
I should also add that I use (roughly) a 50px padding (margin) on the left and right for the desktop, and 20px for the mobile.
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