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.
Ooh I’ll finally have to switch! One less app sucking up battery in the background 😍