If you have an extra disk slot, you simply replace the existing disk with the larger one. Whence the first new disk is working, the old can be removed and you repeat for the next disk / pool.
It is un-common to replace a disk in a single disk pool, because generally it won’t work if their are block errors on the source. But, if their are no errors, it works fine. (I’ve done it, though it was a good many years ago…)
Now if you don’t have an extra disk slot, then you have to determine if you can export one of your other pools and use that disk slot.
Each pool currently only has one disc in each, so I don’t think I simply replace the disk as the system wouldn’t be able to access the data.
Should I add the new disk (ada3) to the existing sharedmedia pool and wait until the data has been copied to the new disk, then remove the old disk (ada0) from the pool?
Uh, a disk replace through ZFS does a temporary Mirror of the drive to be replaced. Then, when the Mirror is done re-syncing, ZFS automatically removes the old disk.
That said, you have to “replace in place”. As in add the new disk without removing the old.