I made this trip (St Malo / Galway / Stornoway / Thorshavn / Reykjavik) arriving in Iceland
It's the right time. I would say that you want to be in Iceland by mid-June.
So, you have time to visit and sail back before August.
Begining August, in Iceland, they start saying "ahh.. that was a nice summer ... " and indeed the weather
turns to the autunm.
If you go through Faeroes, be sure to have a map of currents. It's like chenal du four between each of the islands ...