Being mitragynine ia mu-opioid partial agonist (according to some studies, not all, it seems) it's not like it hits fully those receptors. The most potent mu agonist in kratom it's 7-hidroxymitragynine that it's only in very little quantities in the leaf.
Some people have studied the pharmacodynamics and it seems that in some kratom varieties (think kratom as a TEA (camellia sinensis), different drying methods create slightly different alkaloid ratios, flavours and so...) have more 7-ohm, but... it also seem to depend on metabolism, as mitragynine pseudoindoxil, some "derivative" that it's said to be around while metabolizing mitragynine seems to convert later to 7-ohm....
it's pretty complicated, to be honest, and they don't even know for sure what's going on in the body with all kratom alkaloids (it has more than 40, in lil quantities).
you shouldn't expect a lot of euphoria because it's not an opiATE, but an atypical opiOID (and supposedly partial agonist) There's so many others natural opioids that barely have effects, and also some brutal hell powered synthetic opioids, but it's not the case with kratom. Kratom it's more like buprenorphine in terms of pharmacollogy as it's said to also have an alkaloid that acts as a opioid antagonist, creating a ceiling in effects, specially euphoric effects, in higher dosages. That seems to vary, depending on the person metabolism. For me the euphoric effects reach their top at around 8 grams, then I get nauseaous, but some people have used 20 grams even without tolerance, and they get good effects...
Normally extracts are more potent and euphoric, but they are said to destroy your tolerance to the plain leaf... in anycase you should go to a specific kratom forum where sourcing is allowed, to find proper vendors, as bad leaf it's always very dissapointing and people says that the quality is constantly decreasing since 2017 (more or less), due to enormous demand.