In the past I've read that certain additives can act as preservatives for
juice. Specifically vinegar (white or apple cider) and citric acid (lemon
juice). If not actual preservatives, flavor boosters I guess.
It may be worth it to look into some of the commercial flavor enhancers as well. Vape Wizard, Magic Mask, Bitter Wizard, Sour, etc...
I've never tried any of the above. But I might take a stab at it as my RY4-ish ADV peaks at about 2 weeks and as you've noted, declines around week 6. I'm
vaping some November 1st as I type...