The most logical option for you would be to upgrade to the current Ertiga, which is a generation ahead of the one you currently have.
The current Maruti Suzuki Ertiga is more spacious than your model and comes with a smooth 6-speed automatic too.
However, there is no diesel option available like in the previous generation Ertiga that you own. So if you are looking for an MPV with the low running cost of diesel, the next best option, if you have the budget, is the Kia Carens Clavis diesel. It comes with a 1.5 diesel automatic, coupled to a smooth-shifting six-speed auto transmission.

