That’s why I like the idea of age unlock questsThen you will never be happy with Civ VII and should give up on it. You have defined your problem as unsolvable and thrown up your hands and, by your definition, can provide no solution except an entirely new game.
Some of us, though, do not accept your definition or your problem - which is normal in any game, each of us has our own set of game definitions even when playing the same game in the same way.
In my experience with the game and from reading far too many of the same complaints in these Threads:
The game as designed is built around three Ages and nearly complete revamping of the game in each Age, including the playable Civs available in each Age. Changing Civs is a result of that original decision, and so merely allowing you to play the same Civ continuously will not solve the original problem - the reset between Ages, which currently happens almost entirely Out of Sight of the gamer. You finish the Crisis Period, the curtain closes, and when it opens again the set and the actors are all different and the version of Macbeth you thought you were watching turns out to be a version of Bald Soprano - in Sanskrit, done entirely in rhyming couplets by cross-dressing orangutangs.
It might be fascinating, but it's not what you came to see, or expected to play in a Civ game.
Playing the same or different Civs isn't the problem, the hacking of the game into three parts that do not show enough continuity to the player is what messes with too many player's heads. (And yes, it messed with mine and still annoys the %$#^ out of me, but for me, at least, it's playable)
The question is not "I was playing Egypt: why am I now playing Mongols?" it's more "I was playing Egypt, how the H**l did I get to playing Mongols?"
Answer that question to the gamer's satisfaction, and the rest of the game becomes much more acceptable and playable.
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.