Signs of close to harvest are:
Plant is changing color to autumcolor// fading out
Pistils are orange by 75 percent ( not all strains do it...........like gorilla its hard to say here if you look at pistils)
and here we go... the trichomes...... Get a jewelers loupe // magnifier glass to look at trichomes,,, they should be all milky at least
and from here on ( beeing milky) its just your Choice when to chop the ladies
I let you alink here:
https://www.growweedeasy.com/harvest#supplies-harvest
and yes its possible to harvest the tops, and let the other parts grow longer