Its not 1 particular build. Its any spellcaster using spells they probably would be using anyway. Tashas mindwhip for example would be a specific if boring build. You can use that one spell to naughty word down huge chunks of the MM. Even with "bad" wisdom saves (+1-+6 being "bad") behind the screen I noticed how much worse most creatures intelligence saves were.
See previous statements about Greater Magic Resistance taking care of all of these spells. Sould probably be added to CR 20+ critters and a lot of CR 10+ ones if they are a boss fight. Had a CR 19 Balor get pwned by a banishment spell once it ran out of legendary saves (spam command at it).
Uncommon +1 DC items can be crafted reasonably quickly as well now. My PCs didn't stack them but DCs were 1-2 points higher than theory craft. I think you may be able to get +6-+9 on your spell DC but that is more theory craft.
You're essentially dismissing it because you're not seeing it. Its almost as bad as 3.5 where you have DC 30 or whatever. Its not quite that bad but its not hard to have things fail saves 75-95% of the time.
Spellcasters and outsiders usually have better saves but not by much. And almost everything casts spells in 5.5 now or can do if they want it bad enough. Throwing a hold monster spell or twinning it doesn't require an exotic build or theory craft.
I agree that creatures intended for boss fights do on average need more magic resistance, better saves, more legendary resistances, etc.. They've done a better job in 2024 with that however.
But your average mook? I don't see the need for it. It's OK to let spellcasters shine sometimes with an upcast Command spell or whatever. I find no players I've played with will abuse this tactic. It's too boring to do it too often, but it's fun to do it for some battles sometimes.
As for magic item crafting, that's highly setting and adventure specific. I've noticed from your posts over the years your groups tend to be able to get short rests, long rests, and downtime more frequently than my groups allow for. I'm constantly playing in campaigns with at least some level of time pressure on the party. This drastically changes the tools available to players in that adventure between your groups and mine. It's not that we will never be able to craft, but it's relatively rare for us. I'm not dismissing its impact, but I am noting it's an element of your campaigns.
I've pointed out before how your view of the game seems to differ fairly greatly from the views of designers, experienced content reviewers and video optimizers, and poster experiences here. This does not make your view any more or less valid than any other view and I am not judging you for having a different view than mine, but it does color yours and other people's views of how well the rules function.
You seem to think the rules are not functioning well and everything is easy mode in your games. Many others don't have that experience, though there are some who do share you views. Like for example d4: D&D Deep Dive and his audience, which are focused on one combat per day, full long rest resources ready at the beginning of the fight and acceptable to expend all or most resources in that one fight in a matter of four rounds total. I like D4 Deep Dive just fine, but his one-fight-per-adventure-day assumption is so different from mine for adventures that I don't find his builds particularly useful for my table. I still watch him though, as he makes fun theoretical builds. Similarly, Pack Tactics seems to have similar views to yours concerning downtime for magic item crafting. He's another optimization creator who I enjoy watching but who doesn't seem to have assumptions similar to mine concerning typical adventures.
I'm suggesting the difference isn't experience playing the game, as you've said it is. I think it's setting and adventure assumptions common in your games and more rare in other people's games, on average. It would be much easier to change some of the assumptions in your game about adventuring time, safety of short and long rests, quantity of challenges in close proximity to each other, availability of downtime, time sensitive pressure on adventure goals, etc., than it would be to change all the monsters in your game. But, if you like the set of assumptions you're using, then yes tweaking the monsters is probably your best option.