Great to hear!! I'm happy that it's still useful; I'll confess I sometimes refer back to it myself when I'm prepping for the table!
That's an excellent point about slayer monsters. Using salt on Rock Slugs and hammers on Gargoyles is what I always think of! Wish I'd thought of that when I'd written this originally.
Zee Bashew made a video about "The Witcher-fying" monsters for 5e and that comes to mind. Basically, having a "knack" against the monster to defeat them more easily. As you said, I ADORE it when players get absolutely destroyed by monsters and then come back better prepared; especially if they go and do some research first. Sages and previous victims are great for this and really encourage creating secondary objectives with NPCs or scoping out lairs and habitats first. Good point with Beowulf and the iron shield also! I feel that the weirder and more specific the weakness the better up to a point.