Hi FC, 2 common sources of bitterness are anything 'burned' & tomato. iF the base tasted ok on its own , you have probably burned the spices or garlic at the start of the dish. I've personally never used cabbage in any base & pretty sure I could leave out carrot also. The base is only 1 component of the finished dish, and as a rule personally, if I dont like the taste of it on its own, its not going to go away in the finished dish. The flavour will only intensify when reduced..don't know your location but the difference in flavours can vary street for my next step by street. Im in Glasgow where the chefs/owners are predominately Punjabi/Pakistani & find the taste here is fairly profound..My own curries are like my restaurants except with the volume down low.I;m looking at oil used for frying pakora /bhajis for use in the base & main dish
Regards
ELW