K2 (spice, etc) is the synthetic cannabinoids (https://m.psychonautwiki.org/wiki/Synthetic_cannabinoid), which are often fairly dangerous, but are unrelated to kratom.

Kratom (https://en.wikipedia.org/wiki/Mitragyna_speciosa) is a plant that produces two substances (mitragynine, 7-hydroxymitragynine aka 7-OH) that act like opioids. The pharmacology is relatively unusual, relative to most opioids, as the primary active substances in kratom also bind to serotonin and norepinephrine receptors; other alkaloids in the plant also act as ex. NMDA antagonists which may cause a dissociative experience (vaguely along the likes of ketamine, etc).

I’m not afraid of it, I just don’t have it in me to try again.

The standard lab ranges are defined by “at least some people don’t show symptoms of deficiencies or issues or etc at these levels,” not “everyone is guaranteed to be fine in these ranges.” I was “in range” with B12 until I started developing nerve damage from a deficiency of it, with years of symptoms leading up to it that were ignored because I was “in range.” I was “in range” with folate, because the standard lab tests inactive folic acid and not active folate. I was “in range” with ferritin, and yet had significant improvement from taking more iron. I was “in range” with potassium and then ended up in the ER multiple times from hypokalaemia so bad that I could barely move.

You get the idea. “In range” is not the same as “will get no benefit from supplementing it,” but it’s the kind of thing that ideally is done with proper medical assistance if you think you’re deficient in things.