@OP
which platform are you playing at? (did i miss it?)
the 55s/60s are definately still beatable but imo it´s getting a lot harder there. as you said: just more very solid regs. tableselection becomes more important. e.g. i wouldn´t play a game where already 6 good regulars are seated. it might still show some profit but your edge will be small, the variance high and it will just show a higher EV to open up a 33/36$ sng and play it with an expected ROI which is 2-3 times higher than in the higher sng.
the play against the regulars is kinda easy i guess. you will gain good reads pretty quickly because you are playing with them all the time and then you can try to exploit them. the good thing about solid regulars is that you can guess their ranges pretty exactly after a while. on the other hand they won´t make so many stupid mistakes like the fish obviously 
on)
another important thing is that you are aware of the swings... your edge will be smaller at the 55s -> the downswings will become bigger sooner or later. combined with the almost doubled buy in this might be pretty nasty... dropping 3-4k without making any major mistakes isn´t fun