Yes, you are correct. You will need additional storage capacity if you wish to perform future upgrades reliably. At present, both the operating system and your user data reside in the same pool, zroot.
My own arrangement uses two disks: one pool (zroot) for the operating system and a separate pool (zdata) for user data. Keeping the operating system and data on separate pools provides a clearer separation of concerns and reduces the likelihood that growth in user data will interfere with system maintenance.
For laptop users limited to a single disk, this separation is often impractical. In that case, the simplest solution is to ensure that the disk has sufficient free capacity to accommodate both normal use and future upgrades.
