The Google Photos apps have received an update that should make it easier for you to delete photos. There is now a dedicated button to delete videos or photos when in albums.
Up to now, it was only possible to remove content from albums, which then had to be deleted via the main overview. Now you can also delete directly from the albums. If you do this, the content will not only have disappeared from the album, but also from the main feed.
The function is still quite well hidden, you can find it when you are in the detail view of a photo (where the location and other information are also located) in the menu bar at the very back.