OK. I was thinking about Fred's comments at https://moodle.org/mod/forum/discuss.php?d=225381#p984309, was playing with the Workshop settings form and looked at how other modules do things. Finally, I agree with Fred.
Mixing collapsible sections with advanced fields does not work well. Especially now when we do not even mark those fields as explicitly "advanced" (we just display "Show more" which is not the same). So I decided to stop using them at all.
I reorganised the form a bit so that only the Grading settings section is expanded by default (as it contains the most essential setting for the activity). All other sections have reasonable defaults so the teacher can look at them only when needed.
I improved the section headings a bit so it should be more intuitive now what they contain (very good point Fred).
I prepared a patchset consisting of couple of sequential patches that should illustrate my reasoning behind the change.
Frédéric Massart, can you please peer-review my branch and confirm it's coherent with what was done in other modules. If you like it, I would submit it for integration. TIA.