I'd prefer scaling by era rather than number of techs - it adds a bit more strategy (do I beeline for a tech/civic I really want, or take some techs I don't want as much while I built more districts).
I think the biggest issue is not the scaling but the fact that they can only be made with production. A few policy cards/beliefs, even if limited, would go a long way (I.e. policy card - you can buy commercial and harbor districts with gold, belief - you can buy districts with faith in a city you already have a holy site, etc.)
I think the biggest issue is not the scaling but the fact that they can only be made with production. A few policy cards/beliefs, even if limited, would go a long way (I.e. policy card - you can buy commercial and harbor districts with gold, belief - you can buy districts with faith in a city you already have a holy site, etc.)