CA,
I also was surprised how little of the Akhni stock he used. He knows what he is doing so he obviously thinks it's worth putting that amount in.
The remainder was used in the chef's Masala Sauce that he was making at the same time as the base.
I will eventually get round to editing that video. You may be surprised at some of the ingredients used in that...I certainly was....

I cannot explain why you are getting bitter undertones, the full base has no bitterness at all. If anything I wondered if it was too sweet.
Maybe the base doesn't down scale well....
Or maybe there is something different with the tinned tomatoes you use...
Or maybe me simply quartering the ingredients doesn't work....ties in with the first point...
Or maybe you have done something wrong.
I am open to other possibilities..........
I will discount the last point as you are very accomplished.

I have not managed to try the scaled down base as the curry shed is out of commission at the moment but as soon as I can I will give it a go.
Mick