Level Up (A5E) Cantrip Spellcraft downtime

What's the general consensus on Spellcraft downtime to modify cantrips?

I recently found out (on two occasions) that my artificer (whose only constant source of damage is Produce Flame) was "support only" when faced with creatures immune to fire.

Would Spellcraft to modify a cantrip be the same (cost, dc, ...) as a 1st level spell or a bit less?

I'm guessing like rare spells you'd also get the rare versions of a modified cantrip to keep in addition to the normal ones.

(I'm inclined to go a full "produce element" variation - one cantrip per element - with maybe water causing cold damage, air as the lightning one from the AG and earth as an option for piercing, bludgeoning or slashing damage depending on what form the manifested earth/rock takes)

Anything I am missing?

A cantrip that does magical piercing, bludgeoning, or slashing goes through most resistances and immunities. So I wouldn't allow that as a variant of a d8 damage cantrip.

Produce Acid, Cold, or Lightning do seem appropriate as rare cantrip variants of Produce Flame. Your best option from those is probably Produce Lightning, as somewhat fewer monsters have lightning resistance.

Depends on whether or not you want to just have a separate element, or create a version that partially pierces through resistances/immunity like the Rare version of Firebolt.

You should get to keep the base version of the Cantrip in addition to the modified version RAW, but it's kinda subject to your own group thing I suppose. The only thing I've been disappointed by the Spellcrafting section has been the regards to Cantrips since they're missing from the Table and any questions for clarity regarding them in the ENWorld Discord are met with crickets by the designers. I'd say it's almost as if whoever wrote that section forgot Cantrips existed.

Anyway, my group extrapolated DC 14, 50pg, and 1 week for rare Cantrips based on info in that table, though I've heard that others have weighed in saying that their groups just use the guidelines for 1st level spells.