the manga seemed refreshing until all this noble bullshit, hey, take our family to some random kid because he saved one of my servants!! if that kid was a side character nothing like this wouldve happened
A kid with expert level stats, ability to tame hundreds of slimes, capable of making store-grade potions and above 7 pain tolerance wouldn't be a side character to begin with. It's entirely possible that this family is trying to get on his good side, since he seems really strong to them.
Also about them not knowing slime evolution conditions- It could just be that it wasn't useful or profitable enough (except possibly for cleaner slime which has the most specific conditions) that nobody really wanted to dedicate their lives to researching them, which is why it wasn't known. ie, it is remarkable for a child to figure it out but it's not exactly game-changing research.