Sunday evening car rental

Yes, as you have apparently discovered, many, maybe I should say most, car rental offices in "the Provinces" (anything outside of Paris) are closed Sunday evenings in rural France. They take Sundays pretty seriously there as a day of rest, like the Bible says.
If there is a regional/major airport around there, their car rental might be open on a Sunday evening, if it's early.

Otherwise, you're probably not going to pick up a rental car in Tours until Monday morning.
So book a nice hotel for the night in Tours, have a nice dinner, with a bottle of French wine (since you won't be driving).

Welcome to France. France is worth the hassle. They just have a different cultural attitude about certain things, it's not all about the almighty euro.

And in case it comes up, on Sundays it's hard to find a gas station open in rural France, except at the Auto-Route service plazas.
And you'll drive through rural towns and not see anyone out. It's family day on Sunday, and apparently spent mostly at home.
Again, I'm talking about in the Provinces, not in Paris. In Paris, it's family day but everyone is out.

Yes, rent your car in Paris and drive to Amboise. It's not difficult driving out of Paris on a Sunday and it makes no sense to pay for both a train and a rental car on the same day. It's a short drive to the Loire Valley. If you want to avoid driving in Paris at all, you can get the car at Orly airport, which is on the way to the Loire.

Do you need the car to get to your accommodation on Sunday? The Amboise train station and Europcar office may not as convenient as they could be, but they are options, if you're sleeping in the town.

I have always wondered why people say it makes no sense to pay for a car rental and a train on the same day. It makes sense to me because it is worth some amount of money to some people to drive for less time on any given day. Anyway, back to the point of the thread. You could try combining your train ticket purchase with your car rental. You would be stuck using Avis as the rental company, which is something that I avoid because of their drop-off charges for one-way rentals, but an employee of SNCF will be at the station to give you the care keys. I have not checked to see if Tours is a station in the program but I have no reason to think that it is not. Do you mean Tours or St-Pierredes Corps, the TGV station just outside of Tours? Are you arriving in Tours from CDG, Paris or elsewhere?