I don't use modules but I am constantly keeping an eye on difficulty I expect compared to actual play experience. I then adjust, making the encounters more or less difficult to provide a balance that works for the group.
So if I found that the group was steamrolling every encounter, after the first few encounters I would be turning up the challenge. So to me what could have been informative would have been a comparison of the XP budget in the module to the adjusted XP budget. Throw in rough number of items and rarity the group has and we can talk about challenge levels.
So let's take a look at your post where you list monsters.
XP Budget Cap for 5 level 18 PCs is
Low | 25,000 | Med | 43,500 | High | 71,000 |
Since you didn't include PC levels for level 20 it would be
Low | 32,000 | Med | 66,000 | High | 110,000 |
Considering how much magic you guys have, I'd probably up your effective PC level by 2 or more. But that's always going to be a DM's call and part of balancing encounters.
Fight 1: 2 CR 10, 1 CR 5. XP budget: 13,600 XP. Significantly below the Low budget.
As a general rule I don't throw many monsters that have CR 5 levels lower than the PCs unless I give them some significant advantage. We may have bounded accuracy but it only goes so far. I don't use monsters with CR 10 below the PC's level unless I'm using mob rules.
Fight 2: 40 minions from a book I don't have
You don't give CR and the last time I used minions it was in 4E where I almost never had an all-minion encounter. I will note that they are either in fireball formation or bunched up closely enough to be affected by your cone of fear which makes it easier than it could have been. If I had been DM, these would have been coming from different directions, possibly half on 1 round, half on the second or similar. Also, the ones that ran away never came back?
Fight 3: ?
Fight 4: 1 CR 19, 1 CR 12. XP budget 30,400. Low end of medium.
This is about the easiest I would ever make an encounter, especially since I would consider this almost a solo fight. Personally I wouldn't consider a lone CR 12 monster much of a threat because it's so much lower.
Fight 5: 1 CR23, 1 CR 19, 1 CR13, 1 CR10
This should be a tough fight but you are all basically at full. On the other hand it did take 4 rounds so fairly typical combat.
So assuming y'all are level 18 you had 1 super easy combat, 1 low-end medium and one difficult combat that took 4 rounds. If you're higher level, you had 2 fights that didn't hit the medium cap and only had 1 combat that didn't even hit the "high".
Assuming my guesses at level are correct, I'm not really that surprised by the results you're seeing. A big issue is that you're using 2024 PCs but not 2024 encounter XP budgets. Yes, the PCs are more effective in the 2024 rules, but you're also loaded up with a ton of magic (did you ever have this much in other games?) and the encounters are simply too easy IMHO. You basically had 1 difficult fight at effectively full resources and it took 4 rounds.