Sorry if this is a reiteration of previous posts, but I'm not a fan of Garth's stat drinks (there I said it!).
I think a different system such as meditation or prayer could work well, but I wouldn't be inclined to keep the drink buffs as the player could double buff causing imbalance. Here's a brief example of how I would implement these:
Meditation - A camp based, once a day buff lasting for 12-24 hours that improves the weakest of your stats with the very weakest gaining a +2 and the second weakest gaining a +1. This could scale by level or simply how many times you've meditated, and could have upgrades locked behind the monkey guy in Garth's gym.
Prayer - A camp based buff like meditation (possibly replacing meditation when the player chooses a god/demon/person? to worship) that offers a stat buff in accordance with who you worship. A lot of flavour could be added to this, such as offering gold, sexual favour or even purity in return for the buffs.
My biggest gripe with the stat drinks is that I find it a little tedious to be running back and forth to the tavern, and despite the generous timer it still takes you out of the flow so to speak when you need to return to buff for a tough fight. I also think maxing a stat is perhaps a bit much, and a scaling system that can be used out in the wild is both less tedious and more balanced (in my humble opinion) while also keeping it thematic. This would also make camping more appealing, but I can see an argument for having these as a once a day power to be used anywhere (or perhaps recharge on sleep).
edit: another option would be to pick a deity near the start (with more unlocked through the course of the game) that gives a flat bonus that scales with level and purity/corruption, for example: choose 'placeholder warrior god' and recieve a +2 to toughness that scales based on level and purity. Choose 'placeholder demonic god' and recieve a +2 to presence that scales on level and corruption. Exceeding the purity/corruption limit could make you godless and remove the buff.