I once played in a party where almost everyone had a level or two in warlock. I myself played one, but stayed single class all the way to level 13.
The easy fix is to move Hex Warrior to Pact of the Blade. And also make Eldritch Blast a class feature that scales with warlock levels.
Don´t allow pact magic slots to fuel smites. The multiclass rules can be read in that way. The cross usage is explicitely stated for casting spells, not more. That alone makes the dip a loss less attractive.
Hex warrior on its own is not too great. Even for a paladin, having less than 15 Str makes him slow as hell in plate armor, so the most you get out of it is a +3 bonus later on. If you consider, that hex warrior does not work with two handed weapons without pact of the blade, GWM can´t be used with it. So you need 3 levels instead of 1 anyway. Also eldritch blast is not that great without agonizing blast and the Hex spell! At that point, we are speaking about level 7 when you first notice the benefit when you get your aura. At level 8 you lost a feat, so the difference is lower again. At level 9 both can use great weapons, but the hexadin still has no GWM and also no level 3 spells. Then, finally at level 10, if you took another level of warlock, you finally have GWM, at the cost of level 3 spells and higher paladin level abilities. At level 11 the single class paladin gets improved smites. At that point, The hexadin is better served raising warlock to 5, getting another invocation and level 3 spells, where Paladin and warlock levels are balanced.
At that point, you can ask yourself, what character is stronger. I guess, if you pick smite spells (which are quite undervalued), I guess Hexadin will be about as strong as the straight up paladin, but not more. Most probably there are levels, where one is slightly ahead and levels where it is the other way round.