As the starting production bonus seems to have too high impact on games, but cannot be regulated through starting positions well, two ideas that could be used to fix this problem.
1. Sawmill +8 production no longer benefits from production bonuses.
This is where most of your production comes from, so it reduces the impact of early production while not affecting late game that much, albeit it does make miner's guilds and mechanician guilds as well as inspiration worse.
It might be possible to only remove the effect of terrain instead of all production bonus though which solves that problem but isn't very intuitive.
2. Terrain production bonus capped at 3% / population like terrain/road gold bonus
Simple, effective and utilizes an existing game mechanic. It does make places with lots of mountains bad though as those won't have the population needed to utilize the production bonus.
Or we can
3. Leave everything unchanged, as many other factors have an equal or higher influence on games anyway. The higher food requirement and lower mountain production should reduce the impact of this problem somewhat, too.
1. Sawmill +8 production no longer benefits from production bonuses.
This is where most of your production comes from, so it reduces the impact of early production while not affecting late game that much, albeit it does make miner's guilds and mechanician guilds as well as inspiration worse.
It might be possible to only remove the effect of terrain instead of all production bonus though which solves that problem but isn't very intuitive.
2. Terrain production bonus capped at 3% / population like terrain/road gold bonus
Simple, effective and utilizes an existing game mechanic. It does make places with lots of mountains bad though as those won't have the population needed to utilize the production bonus.
Or we can
3. Leave everything unchanged, as many other factors have an equal or higher influence on games anyway. The higher food requirement and lower mountain production should reduce the impact of this problem somewhat, too.