for EWR, flexible dates and an EF subscription are pretty key.
for PHL, i regularly see at least U1 on most flights in advance and sometimes even U7 if booking far in advance
i have not tried to fly to FLL due to limited U availability and more convenience at RSW on DL for my final destination
IAD, BWI seem to be U routinely available
to go to MCO and DCA i just avoid SEA, going instead via SAN and LAX respectively, more convenient for me too.
most of my bookings are >4 weeks in advance so that helps too