A good tarp, big enough to set or take your tent down under in bad weather.
A decent rain suit.
A nice lite hammock and/or lite chair.
Do a search in the trip reports section.
I solo only, but wouldn't consider myself a "minimalist" solo paddler. I don’t mind taking some creature comforts but if you’re okay with the minimal and eating well isn’t a concern, then you can easily go down to the  bare necessities.
Mad_Mat travels fast and light, so reading one of his trip reports might give you some good ideas.
If you’re doing mostly rivers, the portage/weight issue shouldn’t be too much, so that gives you room to decide how minimalist you want to go.
At the minimal you need enough nutrient dense food to give you the right mix of protein/ complex carbs and good fat. Then you want to stay dry and warm, traveling or in camp/sleeping.
Small cook set or just pot IF you go dehydrated food. Way to purify your drinking water.