I think there is already a mod that turns off all approval gains, but beware that I think this also blocks off several companions’ personal quests, since they’re related to the relationship. At least that’s what I heard.

The mod I’m using tweaks approval so the gains are smaller for little stuff, losses are bigger and important story decisions etc become more significant (in both directions). I unfortunately didn’t find it until I was already near-max with several companions but it should in theory make it more difficult to end up with everyone being in love with you before the third long rest.