the reach problem is real.
bigger screens make more room for maps, messages, and photos. they also make the far corner of a full-width keyboard harder to reach with one thumb.
that does not mean you need a smaller phone. it means the part you touch most should adapt to the hand that is holding it.
three ways to shorten the reach.
reachability lowers the top half of your screen. apple one-handed layouts pull the keyboard toward an edge. una mano keeps the keyboard on the side your thumb can reach and makes that position easy to change.
a better default for your thumb.
try the side that matches the hand you use most. if you change hands often, use centre when both thumbs are available and move the keyboard when one hand is busy.