Old WD Red are tricky: The older ones are CMR, further models are SMR.
You’re lucky here. With 2x8 + 4x4 you have potentially 16 TB of raw space in a stripe of 3 mirrors. The lone Skyhauwk has no obvious good use. 2-way mirrors of large drives are not the safest setting, and you may use only about 75-80% of these 16 TB before you need more space, so 16 TB raw is barely more than 12 TB usable.
6x8 TB in raidz2 would be 32 TB of raw space. 6x6, 24 TB raw. (Assuming a future B450/B550 motherboard with 6 ports, to avoid a HBA.)
2x8 + 4x4 TB in a 6-wide raidz2 is 16 TB raw (6x4 TB), so barely enough for now.
There is no conversion from mirror to raidz2, so you need to decide upfront on the layout. In any case, you’ll need to bring in bigger drives soon.