I don’t have much riding experience out of Shawinagan so I can’t speak much of that area.
Most of my/our riding is not in the trail network and done in back country roads and pathes north of Mt.Laurier and St.Zenon, I know these areas very well and have done a lot of riding up there.
Cabanon would be a good location to do loops out of and/or a 3-4 day trip, Ste Anne du Lac good to start trips from too Rabaska as well good for loops and trips. Cabanon and Rabaska would be safer parking when leaving for a few days all though I haven’t experienced or heard any bad reports of SAdL. In the past I’ve started a few trips out Relais Windigo by the falls but that place burned down a few years back and so we park in SAdL behind the motel and ride out to Rabaska or up to Lunic and beyond, more options there. There’s a new atv/snowmobile bridge built last year crossing the Riviere Lapin so getting to Mekoos on atv from SAdL is now possible with out having to go 10 klms south on the highway risking a ticket to catch a backroad up to Mekoos.
An interesting ride out of Cabanon, follow the sled trail up to the dam then on up to Repos and Kanawata, visit Casey and the runway. There’s even underground bunkers around the Casey runway that are interesting visits.
Out of SAdL or Rabaska head up ch Lepine to Clova then on to Martins on the reservoir Gouin, from there head west, some nice dams and dykes to see and you can return via Balbuzard back to Ch. Lepine, more runways to see over there too.
Good luck with your ventures