call that Diss Spell Magic
According to Matt Colville, and I’m paraphrasing here, “Your monsters should be doing average damage equal to 6, + 3 times their CR”. Therefore, your < CR 13 monsters were so nerfed that their actual CR was less than half (+/-) the printed number. All due respect, but if you’re training your players to interact with the game system in this manner, you’re not only making more work for yourself, but you’re setting them up for a rude fucking awakening at nearly any other table than yours. 🤷🏼♂️
Looking back at some of the monsters I used, which were generally official monsters or from tome of beasts 1 or 2, they traditionally were dealing about 84 damage per round if it hit all attacks but only 21 per attack. I remember using an adult white dragon (CR 13) while the players were about level 10. That’s going to either use it’s breath (54 but 22 on a save), or any other combo of attacks that wouldn’t deal over 20 withousome lucky dice rolls, averaging 13-17 damage per attack, and 58 damage if it uses its legendary action to use it’s tail once and hits every attack.
Also my table sounds like a different experience to yours. All the players are people I’m friends with in person and as we’ve become more experienced with RPGs, were played different games rather than optimising 5e, and we tend towards storytelling focused RPGs like Fate or collaborative storytelling games like For The Queen. If we are looking for difficulty, we’re likely to seek a full horror experience over 5e. I enjoy the content Matt Colville but I know his style isn’t for me, it’s very tactical and all about combat, while when I play 5e, I find combat often becomes a necessary chore that the system is built around that only engaged each player half the time and takes far too long to warrant the storytelling it provides.
Ironically enough, I generally can barely stand D&D these days, and much prefer systems like Blades In The Dark, Powered By The Apocalypse, Ironsworn/Starforged, etc., so please understand that while my criticism comes from a place of cited reference, in no way do I support the tactical “storytelling” that WotC sells to the unwitting and the brand-loyal. In fact, it seems that we’re of very similar mind, after all. 🤗❤️
edit: I’d like to point out that I had the “privilege” of playing in a game that Gygax ran at GenCon back in the day, and Colville’s “style” is cut from similar cloth. Some might enjoy it, but I genuinely wish the flyleaves of rulebooks still assured readers that the book’s contents were merely guidelines to assist imagination among their group.