Most of the photos for DT are on my local machine. But soon, I hope, will start doing timelapses with raws. The raws will be on the NAS and separate collections. My SSD is only 512Gb minus system minus a whole bunch of personal stuff.
Darktable has a “copy locally” feature where you can… Copy files to a cache on your local machine and edit them, then sync the sidecar back to your nas.
The files that you’ve “synched locally” are available to edit, since they’re on you local machine. When you want to sync the edits back to your NAS, turn it on and do so, then the files can be removed.