As I'm sure you know, these are both very different vehicles, but the good news is that each one is very capable in its own right. Both offer petrol and diesel engines, as well as manual and automatic gearboxes, so you should be able to find a variant that suits you.
The Kia Carens Clavis is the more sensible option, with more fuel-efficient engines, a more spacious cabin - even if you only use 5 of the 7 seats - and better features. However, it's not very exciting.
The Mahindra Thar Roxx, on the other hand, is the more emotional choice, with more power and performance, and great road presence. However, its ride is not comfortable, and because of its high floor height, entering and exiting is not easy for elderly people.
Your usage is not extremely high, so you could choose the Thar if you are prepared to live with its shortcomings.

