in my opinion, stiff leg deads/romanian deads, reverse hypers, glute-ham raises and good mornings are all awesome.
even kettlebell swings will get the job done to a certain extent.
as for frequency it’s a big “it depends”, as with most things.
time of season, training objectives, training age of the athlete, injuries… they all come in play.
for my 2 cents, i tend to use a 3:2 ratio (roughly). so if i did 6 sets of a squat variation, i’d do 4 sets of posterior chain work.
cheers!