Hi Marc,
nobody would even check if u have a new bike when coming back to Genève "Le parcours complet - La Route Verte" - it is a really nice route. However, I believe that if you would feel the tax-free return form in France and pay the respective taxes in Switzerland you could save some money. However, nobody would even check anything even if your bike would be made of gold :)
Check Veloland - they have very good deals, specially at the end of the summer - however, with the current situation u may find really special deals....