The default "Keep sorted by oldest date added" is quite annoying and not helpful. I can't seem to unlock sort options for All Photos.
It is intentional. This way you always find the photos you recently added, at the bottom of the All Photos album, even after the photos have been gone from the Last Import album. It is the only place where you can find the recently imported photos quickly. No other album can be sorted by "Date added". All predefined smart albums are having a fixed sort order.
Any album or smart album you create yourself can be sorted by the title or the creation date. If you want to create your own All Photos album, as Larry recommended, you could use rules like this:
File > New Smart album, then set it up with a rule that will be true for all photos in your library:
I'm using these rules:
- A "Date is after" rule with a date older than the date of the oldest photo in my library.
- A "Photo is not Hidden" rule, to exclude photos I have hidden, because hidden photos will be shown in albums and smart albums.

You could also add a "Camera Model" rule to separate the photos from different cameras.
Then ctrl-click the new album in the sidebar and select the sort option you want:
