I'd cut trmhe garden route to 3 days potentially, fly into PE, drive to knysna and overnight, then to Hermanus and overnight then to cape town. With the two extra days consider a night or two in Franshoek.
Also don't underestimate Cape Town. A lot to do and even in terms of nature and animals worth seeing the Penguins at Boulders as well as spending a leisurely afternoon at Kirstenbosch. Also Cape Point and Table Mountain (or Lions head if you enjoy hiking).