Hello JuicyFruit, i can see what your problems are,
if your ph from your feeding is around 6 its way too high at this stage.
I strongly recommend to lower it to 5.8 or 5.7
I think she will bounce back up get bigger buds and also get greener again where the leaves are yellow.
Also to answer your question, there are more than one aspects to look at when your close to harvest.
But to my opinion its best to look at the trichomes, when most of them (around 90%) are cloudy than your ready for harvest.
Do NOT let them turn amber, it will give a more narcotic effect, less taste and a harsh feeling in your troath when smoked,
You can check your trichomes out with a magnifier x40 and above for a good peek.
Hope this was an helpfull answer. Keep on growing my friend. Peace 👍