Unable to backup/upload to iCloud on iPad due to full storage
So I left the iPad overnight and it successfully uploaded everything to iCloud despite having very little available space. I believe this may have something to do with the fact that I manually started a backup via Settings->Name->iCloud->iCloud Backup->Back Up Now rather than using iCloud Photo Library/Sharing function. The backup did complain about not having sufficient space but at some stage it seems to have kicked in, perhaps the device re-evaluated what it was reserving in system storage and made sufficient space.