Have you considered just using the user’s browser font via sans-serif
?
I did this on my website some time ago and honestly feels like it is the best way. You’re repecting the user’s wishes if they have a custom typeface specified, and the defaults on most OS’ are good these days otherwise.
Plus, you save some network requests and page size.
LiFePO4 batteries like being charged up high, so you’re probably fine keeping them plugged in.
You’re right about not going under 20% often, but you’ll want to charge to 100% at least periodically to allow the cells to balance.
Edit: Please make sure you’re looking up information on Lithium Iron Phosphate batteries specifically, they behave differently from standard lithium batteries and a lot of the advice shared here won’t apply.