Vestureofbloods maglite dropins ran fine on 4 D Nimh cells at around 3 amps for around 50 minutes during his testing if i recall correctly…
Edit: he only ran it for 20 mins, but output didnt really drop in that time…
Did you just have a faulty driver? Everything else sounds ok. Its easy to say when its not my money being spent(!) but I say dont give up yet! You could try again with a new driver… maybe check it works correctly without fitting the extra chips first, to save time if it doesn’t?