The number of cycles for modern SSDs winds up being years to failure (like 5-10 years).
As others have noted, mirroring to different partitions on the same physical device is a bad idea. You appear to have redundancy, but single device failure kills you and you lose any benefit of the mirror.
Partitioning: keep in mind all 1TB disks are not the same. Some mfgs may count in powers of 2 others in powers of 10. If you are going to mirror or RAID, I think partitioning the disks is a good thing, you make the parititions exactly the same size. ZFS winds up using the smaller size when you start putting drives together: a mirror of a 1TB drive and a 500GB drive will only be a 500GB mirror. If you partition the 1TB drive into 2 500GB and then mirror one 500GB parition with the 500GB partition, overall performance will suffer.
@jmdavis has a good system; it’s always the data that is important, the OS and programs can usually be reinstalled. I like to separate my OS and my home directory onto physically different devices, makes it easy to upgrade the os. I’ve “evolved” to a SSD for the OS and mirror of 1TB drives for my home directory and other data (I don’t have as much data needs).
Since you (@Jimserac) say this is a desktop, if the existing disks are relatively new (spinning disks fail too), I’d simply get another 1TB drive, use the 500GB to install the OS on, then mirror the 1TB drives. I think that can be done during install, but it’s easy enough to do after install (I’d have the 500GB plugged in as the only drive during install). Once the mirror is set up, it’s easy (not trivial) to move your existing home dataset over to the mirror, change a few ZFS properties and you’re up and running.
If you’re interested in a lot of details, look for the ZFS books by Michael W Lucas and Allan Jude plus Michael’s Storage Essentials. Awesome references to have on hand for everything BSD storage related (my opinion and no I don’t get kickbacks).
A quick internet search for “how long does an SSD last” is interesting. An article on on ExtremeTech from back in 2014 shows a Samsung 840 250GB making it to 9000TB of writes, so if you do the math, that works out to “a heck of a long time”.