For that length on flowing water, if you have the budget I highly recommend the USA. Many canoe trails with good campsites (generally picnic table & fire ring), good outfitters, proper feeling of wilderness and nice wildlife. You’ll need to plan well for 6-7 days and you will need a water filter (e.g. beavers carry giardsis). You also tend to get lakes on that length of trip, so read up on canoeing on open water and when not to attempt a crossing.
Ideas:
Allagash Waterway, Maine. We did the West Penobscot for 4 days and we will go back. Northwoods, moose, beaver, otters, deer, bald eagles, more moose. Best in summer.
Downstream Colorado tributaries Utah, e.g. Green River. Geology and canyons. Better in spring / autumn.
We recently did Okefenokee, Georgia (mostly flatwater) which we loved. By far the quietist / most remote from another person i’ve ever been and full of wildlife (deer, owls, birds, frogs as well as gators). We were recommended a river trip in South Carolina, I think Edisto River trail.
I’ld like to know what other long river options you find. There are also loads of flatwater and sea kayaking routes (i.e. trails with established campgrounds). As well as the properly adventurous / possibly a bit too remote options like the Yukon.