I've been using Linux for over 15 years

Me too. I have also separate partitions for different purpose.

Sweet Home 3D is not seeing my main data partition

Normal. All other programs act the same (Libre Office, Inkscape, kate etc.)
At start are mounted in system ONLY "/" and "/home".
All other partitions have only shortcuts in file browser.
First I need to click on it then insert the root password to mount it in the system.
For many personal reasons I don't like to use sudo.
After that I found it in file chooser (from every program) under:
Root/media/ my_user_name / partition_name
That's applicable for every new reboot or boot.
At least that's the way I use my linux systems.
I use debian with KDE.
Hope is useful.
P.S. An general advice: DON'T use the repository version.
Use the installer version from this site.