probably silly question, going from 2 drives to 1 drive + ssd

[ed]

Yes, in order to have caching, you will need at least one M.2 SSD. One stick will allow write cache. Dual SSD will also enable write cache. Most of the time 2x500GB is all you need for home use.
If you have set up your NAS with a single drive, you can migrate to RAID1 easily.
Here is how https://docs.qnap.com/nas-outdated/QTS4....B255C.html

I hope this helps.