These changes would give dedicated alchemists an option to improve their damage but at he cost of crafting the ammunition needed.
And since fine/finest cuilin stones are pretty difficult to obtain in large quantities, it would be their ammo used for mainly only really difficult content.
wasting a finest cuilin on an elemental crystal with a minor buff would need to yield at least 500 crystals to be worth it. ultimately the problem with your idea is that you have the damage boosts increasing at a static pace rather than a damage boost proportionate to the difficulty of actually aquiring the cuilin stones. on water, for example, with +5, +10, +15 and +20, it's just not worth burning the good cuilins on it. you'd be best just using the low-grades and commons you have no choice but to collect on them while saving the higher-grades for golems. if you wanted to have boosts proportionate to the actual value of the cuilins, we'd need to be talking a damage boost growth closer to +3, +7, +17, and +35.
and even then, it doesn't really feel thematically appropriate to just flush cuilin stones into an oven to make your crystals better, because it sort of ignores what the crystal-making process even entails. the cuilin stones are the golem, not a catalyst for generating energy.
Summon Golem
I think fire alchemy is actually weaker than water atm
if you think fire is weaker than water you aren't thinking at all. even ignoring the critical hit properties it's downright stupid to not recognize that a damage boost applied over 3 hits of Flame Burst is better than one applied to a single hit of Water Cannon.