Add something else to biology, rename the medlab to hospital, rename aqueduct to hospital and move it to engineering and reduce cost.
Your wording of this confused me, but I think I get what you're saying so I'm gonna spell it out a little clearer especially because I like it more than simply pushing hospitals back further.
The total growth assistance from buildings is 75%, 50% from the hospital and 25% from the med lab. One big concern raised here is the slow of pop growth in the mid teens. This point is reached in the mid-game, era-wise.
If you break the hospital in half, and put it into the aqueduct building that could come as early as Engineering, you potentially solve the slow growth issue without having the full 75% come online at Biology. Once you get Biology and build hospitals, the "granary" effect will still rest at 50%.
The main bonus of the early aqueduct building is that growth doesn't shut down in the mid-game and wait for the industrial era. In fact, the growth will probably be pretty steady and balanced across the game.
In sum, you could have an Aqueduct, Hospital, and Medlab store food for city growth at inteverals of 25% each... the sum total being the exact same at is in the game now, the only difference being we get a 25% growth boost earlier in the game than we get now, the increments for the rest of the game (50% at biology, 75% with penicilin) remain the same.
Lastly, if the tech was decided to be Engineering... that'd produce interesting decision making because a player would need to decide between civil service and +1f to rivers or Engineering and 25% food storage per pop. Even if the decision is easy to make, it encourages backfilling to the opposite tech in order to maximize growth, which I enjoy personally because I hate how I can successfully make it to biology with barely having classic/medieval techs researched.