When to apply for postdoc positions if papers will be submitted to journals after submitting the PhD thesis?
Get the applications going now.  After all, you will be finishing up SEP.  It's coming soon.  A lot of places are sort of geared to the academic calendar.  (Postdocs can be a little more come and go, but still things may move by the ac calendar.)  

Try to dress the resume up by listing some papers as "in preparation".  Yes, it's not ideal that your pub list is so anemic, but it is what it is.  Waiting for it to get better makes no sense.  After all you can always hunt again in the fall, if needed, with the stronger resume.  But I wouldn't stop from looking for a gig now.  

Also, it's not at all clear to me why you can't submit the papers now so you can list them as "in review".  This would be normal (publishing pre-thesis) in math, engineering, and hard and soft sciences.  But perhaps this is different in humanities if your thesis will be published as a real book.  

Of course your research may be dependent on colleagues or shared apparatus time or the like.  But in that case, I would not be sanguine about hitting SEP thesis and fall paper timelines.  Things can always go wrong.  Try to carve off pieces you can publish now.