You sat down outside at seven perfectly comfortable. By nine you are pretending not to be cold.
The reflex is wrong
Bringing a real sweater. It gets carried all night, or put on and immediately regretted.
You need four degrees, not thickness
On the arms and across the back, for about an hour. That is the whole problem.
Long, open, thin
Long covers the back. Open never traps heat. Thin adds no volume. A heavy cardigan fails all three.
Bundled is about outline
Put the layer on and look for your own waist. If you can still find it, the layer is right.
One piece, August to November
Wear it open, sleeves pushed up. Do not belt it and do not add a second layer under it.