Have you considered Bern or Interlaken? Either of these towns should have plenty of accomodation options.

Both should be a short train ride and if you've been to Switzerland before you know swiss trains are very reliable.
You can check train times on sbb.ch.

Also, some of the towns along Thunsee might be well connected by train such as Spiez.
And maybe you know this already but hotels don't always appreciate putting all their rooms on booking.com so it may be worth it contacting hotels directly and looking at different websites, including destination marketing websites such as myswitzerland, big chains own website such as Accor.

I've been to Switzerland many times and did the IM when it was in Zurich so I can't give more specific advice, sorry, but that's what I would look into if I was doing the race.