I can now answer my own question : I rented a Yamaha 250 XTZ at Norte Rental in Slata, in north-western Argentina. I clearly recommend them, if a 250 is enough for you. They have this 250 XTZ Lander ABS, and 2 250 Tenere (no ABS).
https://norterental.com/
Here is my comment on Google :
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I here a motorbike (Yamaha XTZ 250) during 4 weeks for a long trip of 4700km in the Noroeste (including 1500km offroad), in June and July 2022.
The experience was very positive :
- Good advices and clear proposal during « pre-trip » exchanges via Whatsapp.
- Good advices about itineraries when we came to pick-up the bike.
- The bike was in good condition, with new tires and fresh oil, well maintened, I haven’t got a single mechanical issue during the whole trip.
- The rental conditions were clear and straightforward, at a correct price.
- And the staff is really nice !
(… not to mention the fact that the Noroeste Argentino is wonderful, especially the northern part of the Ruta 40

My best road trip ever !)