I'm going simple on this and assuming using gross hits of the mage and then everyone can assume potency is that -1 or -2 depending on the spell(maybe higher with amps). Starting from the steps in order. With my commentary.
1. choose a spell, you can amp and all the bells and whistles to spells you know. I'd of limited it here, assuming amps get limited in standard spell casting I'd make the limit 1/2 that, though if they totally redid the test like I suggest below no need.
2. Choose a Trigger. Personally I think it should not effect drain but be what modifies your die pool, and te whole test thing in step 3 should be irrelevant to the die pool when used.
command, act of will poof spell goes off. Minor action. Simple action in SR5 when only one attack a pass was the rule it worked. Minor if you theoretically get a bunch built I think this would get exploited. It should be major IMO.
Contact/time next person to touch it or a time delay. I'd make these 1 higher than command in how much they cost. But, I'd rephrase them to being inert until a command(see command trigger) is made. Once the command is set the next person to touch them or when the time i up they'd activate. As is they are basically unusable, time can work but barely, contact you spend hours to make the thing and no no one can touch it, where exactly do you have hours to make something where this would work. Even with minutes in SR5 it wasn't usable which is why they invented magic gloves etc. With my suggestion you'd turn some water or whatever into a preparation, pour it into a flask, command it to activate and then the next person to drink it pour it over themselves would activate it, time delay, you make it with a 5 minute trigger take it to the location, command it to activate and in 5 minutes it blows. I'm not sold on only command triggers being able to do health spells but maybe there is something I am missing.
Step 3 create the preparation. some basic dice pool examples below.It takes hours in the drain to make. That really doesn't work unless you expect people to only make 1 or 2. Serious min maxers can make a bunch, but I don't think it should be designed so only they can really use this. And if you do have a bunch see the issues with it being a minor action.
Hours also stops you from making them on location, or changing in the field. some of that might be okay, but minutes already handled most of that. You weren't making preparations while being chased or something. You still needed to be free and clear for a decent length of time. but it did allow a enchanter to function by making a couple after he got past security that would inevitably spot and confiscate any preparations. It really should go back to minutes. Make the limitation on numbers be your magic rating. 1 prep per magic max kept by an alchemist. Maybe qualities or focuses can alter that.
example people making it
1. Basic professional 5 magic+4 enchanting
2. Basic PC alchemist mage 6 enchanting+ 6 magic.
3. Min maxed for this PC starting mage 6 enchanting+specialization alchemy+6 magic+rating 4 focus
1. Gets 3 hits nets 1 or 2 + their 5 magic is a dice pool of 6-7, not terrible from 9, they last 2-6 hours and took 3+hours to make so you can;t really make a second before the first expires.
2. Gets 4 hits, nets 2-3+ their magic 6 8-9 dice on the roll, dropping to 8 dice is pretty harsh for your gig on a PC in SR, they last 6-12 hours, you can make another if its a low drain spell but after that they start degrading as you make them.
3. 6 hits nets 4-5+6 magic 10 or 11 dice, ouch down form 18 dice to 10 or 11, they last fairly long though 20-30 hours. You can probably make 2-3 more depending on drain no problem before they begin degrading.
For me I think this doesn't work as the penalty of using alchemy scales against those who are better at it. A flat penalty is more balanced, easier to use and makes more sense. Basically I'd say make your alchemy test vs a threshold equal to the drain adjusted by trigger type/amps/reagents, this would limit amps pretty well and reagents would allow even low level mages to pull off a fireball here and there.
I'd have command -2/touch/time delay-3.(min threshold of 2 or 1) This would represent both the drop in threshold to successfully make it, but also the penalty dice against it when cast. 2 through 3 is roughly what most mages would get hit by anyways when making most preps based on the current rules. but it would not overly punish a mage for going all in on enchanting.
Net hits on the creating the preparation test would help it resist dispelling or break through wards while it was still in preparation form, once cast it would defend per usual as the spell. This would increase the die pool on amped spells a bit but generally just a die or 2. The current rules a x3 amped damage fireball would take 12 hours to make a so I would not expect many of those, but at that level you'd lose 2 dice compared to my system(which also is quicker as its minutes) though you'd have a threshold of 10 without reagents, 8 with them, so good luck making one under my system.
Resist drain: basically as normal, though I wouldn't bother with bumping it by trigger, its downtime I don't care that much.
The finished prep, currently lasts potencyx2 hours, degrades 1 potency, repeat. I really wouldn't have potency effect this. I'd have them permanent with the limit being the number you can keep on hand. if I really needed a duration I'd go potencyx2 days or weeks. Sold preparations would have a use by date, basically how long the alchemist guarantees he wont refill that slot. Assume a alchemist would know when a preparation is used so they can refill it then.