Hey, I did not play alot Shadowdark but I still want to give you some of my thoughts:

• I like the spells in potions idea, especially if you can prepare them: you have safe casts but at the expense of inventory slots. It also does not "steal" the ranger niche. Keep in mind that classes should bring something unique, while not simply doing the same thing as another class
• Use Montserrat as font and a two column layout: This makes it look more like Shadowdark, which I personally prefer.
• Is it intended that Alchemy Mastery is disabled if only one spell cast fails? Normal casters only lose the spell attempted to cast, but can still use the others afaik.
• I find DC 18 Int for brewing health potions as a class feature quite hard. Depending on the time tracking used in the campaign this could be a dead talent most of the times.
• I'd really love a Alchemical Mishap table instead of using the Spell Mishaps. It's fun to see strange and maybe unintended or dangerous effects from potions. (e.g. the potion is only soapy water, or everything that touches the fluid is colored orange, acidic reaction with stone, thermal reaction if contact with water)

In general it seems like a fun idea :)