Given your concern for good ground clearance and budget, the Tata Nexon petrol is the best fit. It has a ground clearance of 209mm, among the highest in the compact SUV segment and some amount of underbody protection and a tough suspension. It makes it suitable for waterlogging and bad roads in the monsoons. 

The Nexon 1.2 turbo petrol is well-suited for your 200 km per month city use, occasional long trips and concern about waterlogging. Several petrol variants fall within the 12-lakh on-road budget you have, and there are discounts available too.

Since your monthly driving is around 1000km, it will be better if you go for a diesel car. If you prefer a manual gearbox, the Tata Nexon diesel is a good option for its good ride quality. But if you want an automatic, you can consider the Hyundai Venue diesel AT. 

The ride quality of the 2025 Venue has been improved significantly, and now it gives a good ride quality on Indian roads. 

The Nexon’s automatic option is an AMT, which has noticeable gearshift jerks and can become tiring on long drives. 
In the smaller segment, the Tata Punch manual or the Hyundai Exter AMT are good alternatives. 

Exter’s AMT is one of the smoothest AMTs in the market.
 

The Honda Amaze will offer a better ownership experience for you. Firstly, it's a CVT, which is smoother and much more responsive than the AMT in the Tata Nexon. 

It’s lighter and more agile than the Nexon too, which as a first-time owner makes it easier to drive and manouver especially in the city. Being a Honda car, you can expect it to be more reliable and easier to maintain. It’s efficient and fairly spacious too, and for your budget, the Amaze is the right choice.

Indeed, the Tata Nexon doesn’t have the best reputation for reliability. However, the quality and service have improved recently, especially for Nexons made at the Sanand plant, which formerly belonged to Ford. 

But it's not just reliability, but the fact that the Hyundai Venue DCT is a quicker shifting and smoother transmission than the Tata DCA or dual clutch automatic. 

Besides the Venue, which has recently been updated, feels a lot more premium and plush, and is definitely the better option.

Expecting 12-14kpl in peak Bangalore traffic from an automatic SUV is a bit optimistic, and if you really want that kind of efficiency, you will have to look at diesel alternatives. 

Consider a Kia Sonet or Hyundai Venue diesel AT for its smooth transmission and refined diesel engine. The Tata Nexon diesel is efficient too, although its AMT isn't as smooth as its Korean rivals.
 

Yes, the Tata Sierra’s A-pillar is completely different from the Nexon’s or the Harrier’s. The Sierra sits on a new platform with unique dimensions and hard points - the structural reference positions used for manufacturing and crash performance. 

The Tata Nexon, in contrast, is based on Tata’s older X1 platform, which traces its roots back to the Indica Vista of 2008. So, there’s no structural link between the Sierra and the Nexon.