We’ve got an almost stock 2020 TRD Off Road, only thing it has are airbags, that we use to tow our 16’ travel trailer. Trailer has weight of 4,300 lbs and hitch weight of 490 pounds and the 4Runner does surprisingly well towing it. You definitely feel grades and will be one of the slower vehicles on them, but not dangerously slow. We live in Northern California and have taken up and over and through the Sierra Nevada mountains so it’s done some grades. Even on those we still pass semis while keeping the RPMs below 5000. In the flats I hardly notice the trailer is there and have to watch my speed sometimes to make sure I’m not going too fast.
I will say if you’re towing a long ways versus back and forth across town moving loads, get some air bags. We have a WDH but even with that the back sags a decent amount. The air bags really helped smooth the ride in the cab and made the whole rig more responsive. It towed well before the bags(we had it about a year before installing them) but did better with them.