Have you checked if you have anything left in the Google Photo Recycle Bin? That is taking space as well till the bin will be empty.
In general Google Photo normally doesn’t use space and it is unlimited if you are using the “High quality” setting rather then Original. The setting is available on top of Google Photo settings.
You can change this setting anytime so that you might reduce the used space of photos already uploaded because they will be compressed by using the button “Save space”, the white one in my screen with Italian text, once you changed the quality settings.
No the photos that you upload on Google Maps are not stored in Google Photos neither Google Drive. You can check more about Back up photos & videos here.
Also, I have relabeled your post to How-tos, as it fits better there.
I had the same suspicion that the photos posted in Google Maps are saved in Google Photo because I don’t use Google Photo at all.
All save, synchronization, backup or other options have been disabled for years and yet recent photos arrive in Google Photo.
I made an experience deleting some photos from Google Photo and I was surprised to see that they disappeared from Google Maps and therefore from my posts remains only the review.