An alternative might be to consider using Google Drive - which provides 15GB Cloud Storage for free (more can be purchased). Install the Google Drive App on your devices - and you’ll be able to enable access to it via your native Files App.
https://apps.apple.com/gb/app/google-drive-online-backup/id507874739
- Download the Google Drive app for iOS from the iTunes store
- To turn on auto backup for photos in Google Drive, go to Settings
- Tap on Photos
- Tap the toggle switch to turn on Auto Backup.
Microsoft OneDrive also offers 5GB free Cloud Storage (the same as iCloud) and auto backup of Photos to the Microsoft Cloud.
https://apps.apple.com/gb/app/microsoft-onedrive/id477537958
Google will likely be the best “free” alternative for you.
It might be seriously worth you consideration to move to the lowest “paid” tier of iCloud Storage - as filling your free storage has implications for other Apple Services linked to your AppleID across your devices. It’s only £0.79/$0.79 per month for 50GB - this being sufficient for many/most users.
I hope this information is helpful in providing some advice and alternatives.