1. Limit the DurationThis is pretty obvious; the fact that the spell was originally unending -- up to dispel magic -- was a primary source of its great strength. It's one of those things where one might extrapolate wizard characters with arbitrarily large entourages all from the 1st-level spell. In fact, as you can see in my OED Book of Spells, I'm in favor of a guideline of no spell under 5th level being allowed to be permanent. My own conversion of the charm person spell there has a flat duration of 1 week, period, which I find to be much more elegant (no special table needed), and still provides some opportunity for subterfuge and surveillance, for example.
2. Give a Saving ThrowOkay, so let's you go the "fight against their nature" route -- then my recommendation would be to give at most one extra saving throw in the specific case of "turning against one's former allies" (either in combat or by divulging secret information, say) -- to indicate that the victim is entirely, completely dominated. There are admittedly plenty of these "Fight it, Bob!" scenarios that pop up in fiction, but in many cases the enchanted victim really does wind up horribly fighting their old allies anyway. So maybe give the caster a strategic choice about either to (a) definitely keep the victim out of the current fight, or (b) risk using them against their old allies. That's a reasonably interesting tactical choice.
3. Slow the VictimThe other thing that I do is to treat the victim as slowed, and getting only half attacks, while in combat. What this does is moderate the great "swinginess" that occurs when a character is switched from one party to another when in combat (while avoiding the argument about whether that's "in their nature"), and at the same time giving the flavor of many fantasy portrayals of enchanted people acting against their own will, but doing so very slowly.
For example, here are a few panels from classic Lee/Kirby Thor stories:
And lest we not forget, ol' Reggie Jackson in the Naked Gun movie:
I think I've made my case.