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.