All the new 5 Gens are actually easy to fly, even the F-35B, the FCS is like have a co-pilot, so I'm rather certain J-20, FC-31, SU-57, like the F-35 and F-22 are "pussy-cats", having a two seat trainer for this level of aircraft is no longer deemed a necessity, but the same really couldn't be said for the F-4, Mig-21s etc, they were heavy and they flew very hot, with a high lift-off speeds and even higher approach speeds...
I don't know about the F-18, but the F-4 and the F-14 had NO flight controls in the back, the USAF F-4 had a throttle and stick, so that it could be flown, but no rudder pedals, so they really didn't intend for anyone to take off or land from the back pit, although at least one F-4 was landed by the Whizzo....