The seat map helps if you are within a few days or a week of departure, if 50% of the seats are open I wouldn't expect them to all fill with BE tickets. For example on UA871 tomorrow it is 9s across the board and the seat map is close to the one that went out today with tons of seats open.

With TPE entry restrictions still in place if OP is traveling very soon they most likely would be able to find a row to stretch out (especially if they buy E+) but that is just a guess. However if OP has the means to upgrade to J and is worried about sleeping I wouldn't risk it