KT - I'd wait until one of the limbs on the 5 year old specimen has reached about 18". They all look a little on the short side to me. At a push, you might get some success with the biggest bit but I'm still inclined to believe the alkaloids will be more concentrated nearer the growing tip. Maybe that's nonsense but I'd rather wait until that piece on the previously cut long stem has reached 40cm so you can do a 'cut and shut'.

Sorry to hear you've been so stressed lately. One thing that will reduce your stress a tiny bit is drawing your benzene rings with three separate double bonds - it makes electron pair movement in reaction schemes a lot easier to work out  (This was an early piece of advice given to me by real organic chemists. Maybe not the bit about stress.)

If you're feeling particularly desperate then the biggest bit should do the job. It would still be best if you could give it a month's conditioning - or longer - in the dark before consumption. Something that has worked for me - and I note that it is also a part of traditional indigenous San Pedro use - is incorporating an element of ritual journey into the experience.

This means arranging to get away from your home space (and away from the city). And it seems to make for a more... special kind of experience - you do, of course, have to choose the location wisely though.