Itunes backups don't get everything. No media files originating from the itunes store are backed up. Some apps are not backed up. Anything that's backed up to iCloud is not backed up with itunes. You essentially have to pick one: online or offline backups. If you keep your phone backed up to iCloud your offline backups are useless without an internet connection.
> https://support.apple.com/guide/iphone/back-up-iphone-iph3ec....
I don't understand OPs problem with cloud backups being charged for? Many people (myself included) are storing terabytes of images/videos in their cloud backups. While I think the free tier could offer more than 5gb, its not unreasonable to charge for larger cloud backups, Android does the same thing.