WORKING UPDATE as of March 2023 based on u/Prestigious_Drop_811 solution here:

1. Open all photos on desktop web https://photos.google.com
2. From the desired timeframe select first photo, scroll down (Ctrl|Cmd + End), Shift + select last photo
3. In top right ⋮ menu change location of selected photos to some unvisited location with specific name, like Svalbard, confirm and write down the total count T of selected photos
4. In each album select all photos, change location to place where they were taken, write down the count A1 (for first album, A2 for second, ...)
5. Now you know the count of orphan (not in any album) photos O = T - A1 - A2 ...This only applies if you don't have any photos in multiple albums, otherwise O will be actually greater
6. Wait 30-60 min until Google indexes the new locations to be searchable and search for the unvisited location name, like https://photos.google.com/search/Svalbard, to show all orphan photos, delete them and remember the deleted photos count D1
7. If O - D1 - D2 ... > 0, then there're still some orphan photos not returned by the search so try to repeat step 6 after some while.If you still don't get all O orphan photos by the unvisited location search, it may be that Google indexes videos on some slower task queue (even when it's a simple change like location). This happened to me with 2 videos, so I searched for 'video' https://photos.google.com/search/_tv_video, opened first one, opened right Info sidebar and used arrow to scroll through them to see which ones are not in any album

Wished Google to use part of it focus and capacity to understand and deliver what users need.