Western Red Cedar is also a good external timber and smells lovely. Got a few spare boards that I intend to fix to a table frame for the garden.
We don’t mill ourselves, but do get a fair amount milled for our own use (10-20 tons/year). We don’t get stuff quarter cut as we don’t make indoor furniture. We do floorboards from time to time, but they are cut as blanks to be re sawn and profiled once ready. Most of the stuff we make is gates, boardwalk, fencing, benches and occasional set of barn doors. Majority of stuff is 2″ and we don’t see any issues. Barn doors are 3/4″ or 1″ thick, but well attached to bracing and ultimately they are barn doors, so we aren’t paying too much attention to dead flat levels, just the ability to keep stuff where we left it.
Short answer: thick stuff doesn’t curl so easily and if in doubt, more bigger nails.