I'd stay well away from Viator, tour operators would much rather you book direct with them. Not only will you get much better customer service, you will get a much better tour. Viator take 20% of anything you pay, but still insist on being cheaper than anyone else - I think you can see the problem :)
Check reviews on TripAdvisor by all means, but contact the tour operator directly. You may have to hunt for a link to operators website, but it will be on their review page. Don't be put off if there are no post COVID reviews, a number of companies no longer solicit reviews for TripAdvisor. Don't be put off if the review page says a company can't be booked online, either. What they are saying is that it can't be booked on Viator.
My must sees are Chenonceau, Cheverny, and Loches. Any two of those and a good lunch and a winery visit makes for a good (but long) day.