I also have GG's, one a still a kid, one milker (not in kid, she's having a year off) and another milker (just in kid, hopefully).
I have found that my GG milkers do best on actual goat mix (though it's a bit pricey) and rolled oats for their morning and evening feed. My older GG does not like dairy nuts. At lunchtime I feed soaked sugar beet shreds, still warm, with a sprinkling of garlic and also caprivite - that way they will eat it. Also then they get some more oats and some dairy nuts (the kid likes them). However I would be careful in feeding dairy nuts to GG's, they tend to get a bit fat on them, and don't necessarily milk better....
At the moment I feed my older nanny and the kid together, as it doesn't matter if they put on a bit of fat, they are not in-kid.
My in-kid nanny at the moment only gets a handful of sugarbeet with the garlic and Caprivite at lunchtime. Nothing else. She is milked once a day and gives just under one litre. She will be dried off by mid-December. I have been slimming her down since the end of the summer.
Everybody has hay-ad-lib of course.
The reason why I am sounding so complicated with my feeding regime is that both my GG girls had pregnancy toxeamia this last spring. I am not sure if GG's are more likely to get it than the larger dairy breeds, but I am maybe also a bit to blame in that they were possibly a bit on the chubby side early on in their pregnancy and with the cold winter we had they were stabled all winter and maybe had a bit more concentrate feed than they should have had... So this year I am trying to keep the one in-kid quite trim, which explains the low milk yield as well.
I do not want to scare you, but just keep an eye on their weight by checking their backbone/condition score them. If they are NOT milking they will do fine on hay and a handful (yes only a handful) of goat mix, with another handful of shreds, so that you can add the Caprivite/Garlic/seaweed if you feel it necessary. If they are milking about 300g of mix (or mix and oats) plus about 500g for every half litre will be enough for them. Some fruit/veg at lunchtime is also good. Your feeding regime sounds about right.