So I figured it out myself by using EXRULE
, the solution is very simple, in the end. Although EXRULE is deprecated with RFC 5545 it's in RFC 2445 and the best of all: Google Calendar supports it.
So the solution is:
RRULE:FREQ=WEEKLY;BYDAY=MO,TU,WE,TH
RRULE:FREQ=WEEKLY;INTERVAL=2;BYDAY=FR
EXRULE:FREQ=WEEKLY;INTERVAL=2;BYDAY=MO