If the drives added to the pool need to be formatted, is there a possibility that it wipes the data on it?

that's what I meant, yes, but you said you have 2 16 TB drives right? at least with ZFS, setting up a mirror can be done only starting with a single drive. It's a godsend.

first, you take the empty drive, check that it's actually empty, and if so, create a ZFS pool of a single drive from it, with zpool create. copy all your data over. you can use rsync, it has a bunch of options for preserving most filesystem metadata, and for printing progress.

when done, check that absolutely everything got transferred, and add the other 16 TB drive too to the pool with zpool attach. doing this will convert the pool with only a disk vdev, into a pool with a mirror vdev of 2 disks.

further recommended reading: https://openzfs.github.io/openzfs-docs/man/master/8/zpool.8.html

you may want to enable compression from the beginning. if you do it later, existing data won't be compressed. media files mostly don't benefit from this. compression is enabled on the dataset level, with the zfs command, if you set it to lz4 (recommended alg) for the root dataset, everything will be compressed that way.

also, when and where did you buy those 16 TB drives? I badly need a few like those