Without upgrading (and instead doing it manually) is not going to work.
You are right in that it is an extra property in the ARM template, although in case of Azure it is not enough to increase the root volume size in the ARM template. It's a necessary but not sufficient, because the OS will not be able to use the extended disc. This is why you have to follow these additional steps.
If you do not upgrade your Cloudbreak version to 2.7 and increase the root volume size in the UI, you will not be able to do these steps, because the physical disc size will still be 30G and the disk partition and file system will not be expandable.