You're not an idiot. Be of good cheer.
But also consider why my Kevlar costs more than buying spools of it through The Thread Exchange; I specced it out for model rocket use. So does Quest.
I don't have any problem with hunting for a bargain; I love to do it too. And I've often gotten caught with bargains that turned out not to be useful for what I had in mind (usually from Harbor Freight, it seems). I guess it's one way to learn.
400 lb isn't a bad number for model rocket use. Test it and see if it accepts glue; a drop of Elmer's will keep the knots from coming untied if the Kevlar isn't coated. If it is coated, swage the loops. The coated Kevlar will come untied no matter how tight the knot is, given enough time.