How have you not found any answers? If you're looking to do Mythic raid progression surely you must have theory resources you consult. Where do you get all your information that doesn't also cover this?
Long story short, use 4p21 and no other tier pieces unless you got absurdly lucky with TF procs. Sim yourself at all times.
Legendaries depend on fight and strategy and include a selection of about 5 pieces.
Old Tier has good bonuses, but the stats are generally too low to compete with good T21 pieces. If you did manage to get some 950+ T20 pieces then by all means Sim yourself to see what's best. But normal people won't have access to that.