EXT4 basically takes care of itself. Defragmentation is only a problem when lots of small and medium sized files are splattered all over a drive. Whenever you put large files (like videos) onto a HDD for storage, the amount of defragmentation is negligible.
Here is a YT video explaining how you can check (and defrag if necessary) a HDD:
Thanks for that
I've got a fair bit of music as well as video, so I'll keep all the bigger video files on the HDD & the smaller music files on the SSD
HTPC: LE 8.2.5 / intel i3-7100 / Gigabyte H270M-D3H / GTX1050 / STX Essence