Have done a couple of trips on Glenelg over last year or so, one from Nelson, paddling upstream, another starting at Pritchards and travelling down to Nelson. The campsites are great, especially those with no road access.
Last year the river from Dartmoor was quite overgrown, lots of logs right across the river and difficult to portage, so didn't paddle from there, but with all the rain recently I would expect a lot more water, so could be easier.
If you plan on visiting the caves, be aware that they will not run a tour for just one or two people. I spent two hours waiting for someone else to come along, then gave up and paddled on, yet to see the caves.
Other river users seem to be mindful of paddlers, had only one hoon trying to capsize me in one of the ski-boat zones, others gave me plenty of room.
Met a couple of family groups out in Canadian canoes, having a ball, taking five days to do the trip. Can be done in one day if you want to push the pace, but much nicer to take two or three days to just enjoy the peace.
Recommend this trip for lovely peaceful paddle with some interesting scenery, expecially around the limestone cliffs, quite suitable for inexperienced paddlers if accompanied with others. There is a noticeable tidal push, and can be difficult coming into Nelson with incoming tide and head wind.