It's scaring me how similar your situation is to mine! I also just finished scanning in a bunch of photos that my grandmother took. I chose to host the photos in the Photos app, and considered for a long time whether I would let that sync up to iCloud. Sure, the photos would exist on Apple's cloud. But if I die, they can only be accessed from my Apple devices. If someone can't get into them for any reason, they're as good as gone, because Apple -- as good a company as it is when it comes to customer service -- can't be counted on to let anyone else into my account to retrieve data.

So I stored them in Photos, and will store copies of them on my NAS, in hopes that having them in multiple locations will increase the chances that someone else can access them. Same thing goes with my data -- I ignore iCloud, but I store that data on my Macbook Pro, inside of its periodic backup, on my NAS, on the backup of the NAS, and potentially in the future, on a thumb drive. More locations means more chances of being able to get at the files in the event of a catastrophe.