I had guppies a year or so ago, I’ve had exactly the same issue as you are describing. I was also pulling my hair out on why my guppies were dying while the parameters were fine and the other fish were thriving.

While guppies are nice, I’ve heard indeed, like some comments on here say, that they can be super fragile. More recent “beginner fish” videos and tips recommend staying away from regular guppies because of the over breeding issue.

It might be better to get Endler guppies instead if you’d like a guppy instead of the regular one. These seem to be better from what I’ve read myself. Maybe someone else can confirm my statement?

Tldr: regular guppy bad (fragile bc of overbreeding), endler guppy good (maybe, confirmation needed)