Well I disagree with about everything you say here.
Because there is no way to calculate CR when adding magic items to base monster.
I don't know what you are talking about, this is right in the DMG. I've done it many times. Of course what I find is that you don't need to adjust CR at all if you are matching the magic item capacity of the PCs.
If you don't want to use the monster's items as treasure, don't use the monster.
This is an interesting comment because the correct monster selection can solve pretty much all the issues you list below. So you are advocating selecting the correct monsters on one hand, but not the other.
If you don't have monsters with items, you can't have many warrior monsters that challenge PCs over ~level 8 because:
- they don't deal enough damage
Use stronger monsters. Basically higher CR
- they don't have enough toughness.
Use tougher monsters. Basically higher CR. However, in D&D Beyond you can filter monsters HP and AC. So a pretty quick procedure.
- they don't have decent ranged, AOE, and melee attacks
Use monsters with decent attacks
- they don't have additional movement types
Use monsters with different movement types. In D&D Beyond you can filter monsters by movement type.
- they don't have resistances and immunities
Use monsters with different resistances and immunities. In D&D Beyond you can filter monsters by resistances and immunities.
You can achieve all of the above by selecting different monsters. Or you can add traits and abilities to existing monsters, or you can add magic items (which is functionally the same). All of these options are really about selecting the right CR monster for the task.
Now, if you want to use magic items to achieve those goals, that is 100% OK, but you don't have to.
BTW in MMOTM, almost all the humaniod monsters above CR 5 have magic items. The duergar, giths, and and such.
I don't know what MMOTM is. That being said, I am not against giving monsters magic items, but I personally prefer to keep that with the DM. With one exception, named NPCs / unique monsters. Like Orcuc should probably a more magic items than just its wand. Individuals in adventures should have magic items, but keep them out of the MM, at least IMO.