I might be hearing it wrongly, but I’ve noticed there are actually more than two types of ch sounds:
strong (like in Hebrew), e.g. in nach or Woche
weak, e.g. in ich
/x/ or /k/, e.g. in nächste
like in the English word Charlie (?), e.g. in Mädchen, at least when spoken at normal speed (this one got me really confused)
a (weak) /ch/ sound (not a ch), e.g. in almost any word ending on g like vierzig.
Can someone please clarify? Which ones are indeed correct? Are there unofficial rules (such as: after /d/ it always sounds like /k/). Historically, was there a different writing for each?
I am primarily interested in standard German.