Alchemical Casting - hate the name; it's "alchemy" in a very abstract sense, not one most players are going to associate with the word.
And while I don't hate the actual mechanics of the feature, the properties seem more fitting of a war mage type subclass, not a bookish Lore Master. I mean, fireballs and lightning bolts with a range of a mile? Sounds like fantasy artillery to me. Just send in the Rogue Scouts as forward observers.
I think that is down to their examples more than the actual mechanics.
But also, this is precisely how I would want a "magic savant" character to work. I would want them to be able to change the formula at will to make the spells do different things, and create new formulas on the spot to do what they want, which is effectively what this allows. The only thing missing is a system by which they can memorize modified versions of spells (read, new spells) and cast them without using an extra slot.
Perhaps they can, after using the feature in a specific way:
make an int check to remember for later what they did differently and how it worked, against their own spell D.C.? D.C.+spell level?
Then spend time and resources to develop the spell as if learning a new spell, but longer. Might require downtime. Then, they can add the spell to their spellbook as if learning it from another wizard.
Lets say, spell level is equal to the base spell that was modified, plus the 1/2 (rnd up) the spell level of the modification used?
Also, doesn't it seem like we have all the tools needed at this point to put together a general system for creating new spells?
In fact, my only problem with such a system for Lore Masters is that I think any caster, especially any wizard, should be able to do this, and LM should just be better at it...idk.