Man even GH has PH buffering don't fall for the "PH perfect" scam. You can't afford to just rely on the nutes ever especially while you're learning to grow IMO, and l think that you've proven it to yourself.
On the other hand, size of the pot becomes a problem when you see sings of root bound (Chlorosis), you don't have that going on. If you're set on rootbound and think you need to transplant to a bigger pot, DON'T, just rise the plant a bit and add perlite to the bottom, there's always extra space in the top as the ground is settled from past waterings. Shake your pots, always keep it loose, it a healthy practice. If your finger can't go through the earth on the edges then definitely it's too crowded for the roots and you need to intervene.
It could be Ca-Mg related like suggested but not a burn, adding a supplement or doing drastic changes this far in the game could be detrimental for taste and resin production and yield. So try not to push it... Good luck