It would be considerate to give the participants of your Shared iCloud Photos Library an early warning before you delete it - you may want to send them an email a few days in advance. So they can make plans how to save their shared photos and the photos hared by others that they want to keep. Perhaps they will now need a subscription for additional iCloud storage.
After I cancelled my own iCloud Shared Photos library I had to wait for half a year to get the iCloud storage back and to be able to downgrade the iCloud storage plan. Even after the Shared library was empty, there were still 400GB of additional storage used for a snapshot of my personal library on iCloud Drive.
This used to be described in https://support.apple.com/en-us/HT213248:
When you join a Shared Library, iCloud captures a one-time snapshot of your photo library database and associated metadata and stores it for up to six months. This helps with a smooth transition of your content to Shared Photo Library. Any photos that were in your library at the time of the snapshot that are subsequently deleted are also stored in iCloud for up to six months.
Apple has removed this paragraph from the current version of the document. So I am not sure if this snapshot is still created, but do not be surprised it it will take some time, until you get your iCloud storage back after deleting the shared iCloud Photos Library.