About setting defaults: In the preferences section (little wheel top right of the screen) you need to select the processing tab, from there you can set your preference for workflow defaults.
- scene-referred - the linear rgb way of doing things,
- display referred - the Lab way,
- none…
Option 1 auto sets exposure and filmic, option 2 sets base curve. They also set the module order (ignore that for now).
If you are willing to read and watch some linear workflow related material I would recommend the following:
- darktable: rgb or Lab This is a must read in my opinion.
- darktable 3 for dummies This one and the next are nice examples of the workflow.
- darktable 3 for dummies hardcore edition
The above three are here on pixls.us
Aurélien Pierre also made a set of, somewhat technical, videos that might interest you:
EDIT: Just realized that the above filmic video is for the “old” one. It does explain how it works and the basics, so I would give it a go. This next one is about the latest one:
For those images that already have the base curve applied: You can select all those in lighttable view and select discard history in the history stack module (on the right side). BE AWARE: This will remove all the edits made, not just the base curve!
I would advise you to get a better feeling what it is you want/need from darktable first and start making bigger changes once stuff starts to fall into place for you.