There is no way to exclude an Item from receiving a command sent to it’s group. So you can either do as you describe or instead of commanding the whole Group with one command trigger a Rule and filter and loop through the Items and send a command to each one individually, excluding the one that you don’t want to receive the command.
So I can turn on g_downstairs_light_day, but which has all of them in…and then turn off g_downstairs_light_night which only has the ones I want turned off.
I knew there would be a bright spark here that could give me a way to do it, or give me some aspect I didn’t realise/remember. The multiple group thing I hadn’t thought of.