I did it!
Shift dates for photos in albums. Act on the remaining photos as desired. Shift the dates back again.
Open your albums one by one. For each album:
Select every photo (click first photo and SHIFT + Click the last photo)
Click kebab menu (three dots)
Select Edit Date and Time
Shift dates and times
Change the century to 10xx: If the latest photo was 2019 change the century to 1019, change an album with latest photo of 2008 to 1008. If you have 1999 photos then work it out yourself :) )
Preview. Google shifts all dates and times by the same amount so they stay in order.
Save
Repeat for all albums
Now all photos in all albums are at the very bottom of your timeline. Any that did not get shifted are not in an album, and at the top of your timeline. You can now add them to existing or new albums as you wish - do it left to right top to bottom and they all get added/ deleted / left out of an album deliberately.
To restore, go to the bottom of your timeline for the 1xxx dates (may be your whole time line if you have date shifted all photos))
- Select all photos in the date range of 1919 (or whatever you chose) and earlier.
- Click kebab menu (three dots)
- Select Edit Date and Time
- Shift dates and times
- Change century back to 20xx
- Preview. Google shifts all dates and times by the same amount so they stay in order.
- Save
So there is some manual work, but only as many operations as you have albums + 1 (restore). Please proceed with caution to make sure you don't mess up all your dates. Maybe you can do a bulk copy/ backup or something first.
Please comment if this does or does not work for you.
P.S. Sometimes you add new photos to an album and it doesn't sort them by date so you can't redate the new additions. You can fix this, although many threads say you can't. Go into the album, click the kebab menu, "Edit Album", two up/down arrows appear top right - click these and you get the choice to sort by date (or recently added - also useful).