I did it!

Shift dates for photos in albums. Act on the remaining photos as deisred. 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 hamburger 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 into each album and 

- Select all photos in the date range of 1919 (or whatever you chose) and earlier. Do not select all, in case there are some new ones you just added that still have the 2019 or other original date.
- Click hamburger 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
- Repeat for all albums

So there is some manual work, but only as many operations as you have albums x2 (there and back). 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.
Q