Spells only have saving throws if their descriptions say they do. So if it says they do a wisdom or constitution saving throw, then it matches up against my calculated DC? This is incorrect. You can only critical on spell attack rolls of course, and when you do you just double the dice you roll for damage.

What spells require saving throws?

But saving throws are required for many things that aren't spells, including non-spell monster abilities (a medusa's Petrifying Gaze, for example), traps, poisons, and environmental hazards.

Do Cantrips have saving throws?

A saving throw cantrip has the same save chance even if the caster has 3 levels of exhaustion. An attack roll cantrip, not so much. Keep in mind that they're not really meant to be balanced, especially the ranged cantrips.