I have a very similar setup on one of my PCs. I use the HDD for /data. And that holds all pictures (photos), videos, music, etc. I also have a /data/tmp directory where I put temporary stuff like work in progress.
A nice thing about Linux filesystems are links. You could relocate the biggest directories in $HOME like ~/downloads or ~/documents to your HDD, e.g. /data/Nocturn_Adrift/Documents, and link this to your $HOME. And if you dont like links you can use “mount --bind”. I use links and binds.
But then dont forget to include /data/Nocturn_Adrift in your backups.