I’ve done it and variations twice now. We did it in 5 days so very doable but it’s a strong schedule. consider using the ferry to get from Geneva to the other side, it’s a fair truck round by road and stage 1 is quite long enough already thanks! it costs around £15 for body and bike.
We got snowed on this year in early September and had to make a detour over the Col de Madalaine to get to Valloire as the Col d’iseran was closed from Bourg St Maurice.
This guy has suggested a good schedule and some nice detours, it’ll be pretty tough still though http://www.cycling-challenge.com/category/route-des-grandes-alpes/
Definitely do the Col de Pre to get to Cormet De Roseland, it’s a bastard but the view over the valley is awesome.