I generally recommend against having a combat “system” unless the combat system is the central point of playing the game. If you’re considering whether to have a combat system or not, then presumably you have a game that isn’t essentially about combat, upgrades, and optimization, so don’t have it.
Instead, include combat-related choices that are about the main “choice themes” of your story. For example, if lots of your non-combat choices are about how far to self-sacrifice, put those choices into the game (“take a big risk to protect your team?”) If you have a bunch of choices about honor, offer the player a chance to “cheat” in a one-on-one duel. If your game is about the brutality of war, “do you slaughter the enemy or take prisoners (who may attempt a jailbreak)?”
Choices like those are usually a lot more fun than attack/magic/item/run.