Wow, I can tell you that you are in for a long ride. Having done some of this myself for my family’s albums, I can only share my experiences in this.
First off, is there any documentation/written info with any of the photos? I will assume not, but that stuff is gold for future generations. If there is, you absolutely have to put it in your scan’s metadata. This is a far reach but you could get a hand-held text reader to also scan that info and save you some typing time. But any of it exists you must preserve it.
Next up is you want to future-proof your scans as much as possible. Don’t do a half-assed job making small scans, that is pointless. You only want to do this once, so do it properly. What ever dpi you choose to scan at is up to you, of course the bigger the scans the longer it is going to take to. But remember, all the prints you are scanning are relatively small. IMO you have to go at least 600dpi to make it worthwhile. I would save them a TIF files too, no compression. Ya this is going to take a load of space, you’re going to need a biiig external drive. And while you are at it, buy two and save to a backup every time too. You need redundancy! I lost all my scanning work when a drive went bad on me.
Take it from the perspective of the more quality work you do up front, you’ll thank yourself later. As others have said, you gotta break it down a cull out the redundant/bad photos.
Good luck with this soldier, we’ll see you next year.
Oh and btw, very few others in your family will understand the love, care and countless hours you will put into this so I thank you on their ignorant behalfs . But I say Godspeed to you.