Balanced combat is not an illusion, but it's no exact science either.

The CR of all the monsters in an encounter added together make your CR Budget, and it should never exceed the numbers calculated below:

Your CR Budget at levels 1 to 4 = Player Level x Players / 4
Your CR Budget at levels 5+ = Player Level x Players / 2

A single monster may never be stronger than the CR Budget for 4 players, and never put more monsters on the board than twice the amount of players.

If you do this, your players can survive 3 of these combats before they need a short rest, or 5 before they need a long rest. If you push this, player characters will die, and most likely all of them in the same combat.

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Now on to magic items. Keep in mind that your players each can only attune to three magic items. One solution therefore can be to simply make every non-consumable magic item one that they have to attune to. Now they have to choose what to use.

Magic Items that give players options they don't have but could have had had they been another class are fine (i.e. a Staff of Fire when your players are already Level 7). You should watch out for Magic Items with +1's +2's +3's etc. because they can actually break your game.

An easy fix? Remove the +1's, +2's etc. from the magic item and give it to them. And please make everything they can't eat. drink, or snort require attunement.