Let me just start by saying I think this is an awesome class feature the devs have added. Unfortunately it doesn't seem to be seeing a ton of use, as only around 5-6% of the playerbase has enough levels in paladin, blackguard, or liberator to qualify. It is also unevenly given to these classes, paladins need 21 levels, restricting them from making some of the better builds, but blackguards only need 17 levels, and only 16 for liberator. This means liberators and blackguards get access to more build variety, mainly being able to pick up enough fighter levels for weapon specialization, and weapon master levels for increased crit range/multiplier. Paladins on the other hand can only choose one or the other, despite the fact they already sacrifice more class features than liberators or blackguards do.
My suggestion is simple, make divine synergy require the same amount of levels for any qualifying class. Personally I think 16 levels should be enough. Paladins are already sacrificing epic oath bonuses, spell slots, and valuable CL if they multiclass, Forcing them to also sacrifice divine synergy while letting liberators and blackguards keep it seems unfair. Divine Champions should probably get divine synergy also, since using divine shield + divine might + divine wrath is such a huge wind-up time, it could stand to be whittled down by a turn.
And yes, I'm aware that blackguard/liberator are prestige classes which is why they require less levels for divine synergy, since they can only take 20 class levels max, but most players are just taking fighter levels to qualify anyways, which means they benefit greatly over paladins since that also gives them the extra feats to qualify for weapon master on top of weapon spec. It doesn't even make sense thematically that chaotic or evil divine warriors somehow get their divine synergy faster than lawful/good ones. Should we all just be chaotic or evil since those gods seem to be more powerful?