I've thought about this a decent amount and I also agree that a hybrid system between 5&6 is best. Other people here have thought much more about this that myself but I'll throw my opinion into the mix.
In a nutshell, code of laws should immediately force you to pick an ancient/classical era government. In each government you'll have the choice to go down 2-3 certain policy trees. These trees overlap so that a policy tree like tradition is available in at least 2 types of government. Like they are in VI, governments all have pros and cons but this should be much more pronounced...and a different discussion! Anyways, the policy tree bonuses would be accrued from generating the in-game currency of culture and would operate more in-line to V; non-transient buffs that help you often throughout the game. This is similar to what pineappledan described a while back which was hilarious to me since years ago I've mentioned something like this-good minds think alike!
What I'd like to see in addition to those static policies are "at will policies". These policies would be tied to the science tree and once unlocked, could be added to your government for a one-time reduction in culture. They wouldn't be nearly as constantly available as policy card are in VI; only ~5 an era would be available. But, they provide big bonuses and additional ways to play. Each government should be able to adopt a certain amount with more isolationist governments (Which have really powerful adoption bonuses and the best policy trees) being able to adopt less of these flexible at will policies while democratic governments (Which have maybe less powerful bonuses/trees) being able to adopt more. This would make it so that you're not locked into only a certain set of trees a'la Civ V and can pivot if needed to other playstyles that those governments/trees may facilitate better. Once you've filled up your at-will policies, you could swap them out with new ones but you shouldn't be able to go back...one interesting idea could be to "conserve" a really powerful early game at-will policy for a timing push of production, science, faith, etc. instead of using it when it initially is available. Might add some strategic depth. Lastly, certain wonders should allow you to either a) re-use an old at-will policy or, b) allow you to adopt an additional at-will policy to any government.
It also allows you to entirely forgo these at-will policies if it's better to wait for the natural progression of that social policy tree. If you want to sit back and hit a bunch of powerful governmental social policies (Ones that are more important for consistent growth, production, happiness/amenities, etc.) and not waste culture on the often-smaller bonuses of the at-wills, that might be a better strategy in certain governments or scenarios. It just gives you a lot of options and is IMO a lot simpler for a AI to be able to utilize than the card swapping min/max that we have now. But it also rewards people that can find the best synergies with both types of policies as well. Either way, I think that this system is the best of both worlds; streamlining gameplay decisions to avoid bloat while allowing for a bunch of flexibility with certain governments, policy tree, and at-will card bonuses. Best of all it gets rid of the civic tree and the constant reslotting in VI which Ive grown so tired of!