That’s why I like the idea of age unlock quests
When you unlock a civ, you should get a quest where the reward is culture for that civs unique civics if you choose them in the next age.
The narratives of those quests (as well as narratives presented when you civ switch) could provide a much smoother transition to a new set of bonuses.
My personal preference would be to do away with the artificial and Eurocentric Age system completely.
But that ain't going to happen unless someone takes a digital flamethrower to the game code.
My fall back preference, so to speak, is to make the Age Transitions part of the game play, so that Melvin The Average Gamer knows exactly why he is being given the choice of playing Borussia or Burgundy in 'Exploration Age' and not being allowed to keep on playing his beloved Udmurts from Antiquity. And, of course, part of this is that the potential Choice of playing the Udmurts right through to Space Flight should always be there in addition to a varying number of other choices of playable Civs - unlocked by in-game events and gamer decisions that are transparent in that there should be no doubt in Melvin's mind what led him to playing the Burgundian Netherlands in Exploration Age instead of Spain.
The big hurdle to this lies not in making all Civs playable in all ages - that is a lot of detail work giving each Civ in each Age some playability even when outside of their Primary Age - time-consuming for certain, but not requiring any particular brilliance to achieve. The problem is keeping Melvin from gaming the resulting Crisis mechanics now that they are all visible to him and steamrolling the AI civs from start to finish. We've already seen that despite the efforts to avoid this in the Age reset system, that aspect of the game has failed completely in its stated purpose already and will only get worse if steps are not taken to slap Melvin down somehow.
That means the Playable Crisis Period cannot be a set sequence of ascending crises as now , which always hits the AI harder than the human (just yesterday in an Antiquity-Exploration Crisis I observed several of my AI opponents sitting with 0 total Happiness while I had +60: guess Who was losing settlements to flipping and so entering the next Age appreciably weaker than they ended the Crisis?).
I suggest, in fact, that the Crisis period present as a set of Choices (Narrative Events) which, combined with the in–game situation, lead the AI or human gamer into a state where there are NO completely positive outcomes. That would, after all, be a much better definition of Crisis than merely reacting to a set sequence of events that you know can only get so bad before the Age ends. Instead, the Age does not end until you are backed into a corner: choose to keep playing the Udmurts even as many of them are breaking away (and taking their Settlements with them) because a new religion has reduced their loyalty to -100, or choose to play the Burgundians even though they are not Real Udmurts and so, as an example, cannot build the Sauna UI any more.
The Crisis Choices, then, would lead to the official End of Age which coincides with a Requirement to make a choice of what available next-Age Civ you will play - and that choice always includes your current Civ, even if that choice is not optimal.
Allowing both 'continuous Civ' and 'Civ changing' play is, I firmly believe, the path to making Civ VII a far better game than any previous Civ, none of which offered the same choice of play: if you started in 4000 BCE playing Udmurts in Civs I - VI, you were stuck playing Udmurts even if all their Uniques were related to Tundra (they were a Finnish tribe, for those who have been wondering) and you have been playing on a tropical island map: allowing transitional Civs based on in-game events and choices makes a far, far better game.
Makes you wonder why Civ VII didn't offer such a choice in the first place . . .