I think they could still do a European based TAR without waiting much longer, but with a few tweaks... First, start it in Europe. Get everyone vaccinated and over to, say, Stockholm or Oslo, somewhere far north, quarantine for whatever period necessary, and then start the race there. Would anyone really care if we miss the initial run to the airport from the US departure city? I wouldn't.
From that point, it's land/water travel only. No airplanes. Just zig zag your way south... Oslo to Copenhagen, to Hamburg, to Amsterdam, etc., ending down at Gilbraltar. Assuming the borders, are