sureshot :
you say that farms and mines are equals to camp and pastures..
I think that's not really true :
-farms need fressh water tiles : close to rivers, at least until the good tech and then, you still need to maintain the irrigated chain. and never on hills.
-mines are limited to hills. (and ressources)
-cottage begins low and don't provide any food or hammer bonus.
fishing boats, camps, pastures and quarry are buildable everywhere in your mode and thus have an avantage over the 3 previous improvement. so making them having a bit less yield is not dommageable IMO.
I think in your mode, with the value you give, for all early game you will mostly have only camps and pastures... and maybe quarrys, few farms, few mines, few cottages.
and then, in late game, when mines are +4 and farms +3, workshops are+3, all that was done before will go toward mines, farm, workshops..
so the landscap will change around mid-game.
to counter that, maybe you can make camps... less powerful at early game, and then a bit better late game, (maybe pasture gets bonis with the horse line of techs) but always a bit less powerful than mines or farms as they are more versatil. thus you should have landscape with around 1/2 or 2/3 with cottage-mines-farms and 1/3-1/2 with camps ..Etc.
(and what about workshops, lumber-wind-water mills ???)
maybe if you made them on a 2 level improvement :
pasture (+1F) worked 20/30 turns goes into "corral" (+1F, +1H)
camps (+1F) worked 20/30 t goes into "hunter reserve" (+1F, +1c)
etc...
(but then ,you have to make corral and hunter ereserve able to harvest the ressources... maybe not easy to mod)
thus they would still have a +2 yield, people will be reluctant to erase those improvement they had worked for. and they won't be overpowered compared to early farms/mines, they would be choices.