Put the photos in a container/box etc. and put it in a place with no moisture or humidity. AND, put desiccants in each box to absorb the moisture. And replace the desiccants as needed.
And, put the photos in acid-free folders or envelopes or paper.
To properly archive and keep photos, it needs to be in acid-free containers and wrappings.
And/or in acid-free photo albums.
So that the photos do not degrade over time.
Just put them all in a box, maybe per family or year.
So, for each family, put all their photos in one box and within that same box, perhaps by year. Using file tabs. Because to file them into smaller chunks of "occasions" will take WAY too much of your time. Of anyone's time for that matter.
And per the photos, do not put your fingers on them. The skin oils will degrade the photos over time. Even if you can't see it now.
And do not use ball point pen inks or Sharpie pen inks on the photos itself nor on the back of the photos. That will degrade the photos too. Over time.
Use pencil.
Scanning ALL of the photos, will take... hours and hours and hours of time.
And if you have some professional do it, it will cost tons.
Just, keep the photos, in a box that is intact and sealed, and kept in a low humidity low moisture area/room/closet.
For the photos that are very old, you can also just take a photo of the photo too, then store it on your computer. And back it up on a thumb-drive.
Or only scan the very old ones or historic ones or heirloom ones.
Or yes, give each family their photos. So that THEY can do with it what they want and organize it, themselves. If they care too.
Unless you are the designated family photo historian.
I took an archival course in college.