After years of investigation you need to get the heat source correct. If you take out the seeds a Jalapeno will still be hot. If you blanch a Jalapeno then remove the thin layer membrane from the inside of the Jalapeno then the heat is gone but the flavor remains.
Habaneros are the best with ceviche. Not sure if y’all are familiar with what that is but is kind of like a shrimp cocktail, normally eatin with tortilla chips.