The Nissan Magnite CNG is available with a 1.0-litre naturally aspirated petrol engine, which is barely adequate to power this car. Furthermore, while running on CNG, performance will be lacklustre, and efficiency will also be low. And as you mentioned, resale value will be poor, hence it is best that you look at other options.
The Maruti Suzuki Fronx gets all the basic safety features like ABS with EBD and ESP, which prevent an accident from happening in the first place. Additionally, there are 6 airbags to keep you protected in case of an impact. Do note that the Fronx’s petrol engine is much smoother, and overall, it is much nicer to drive.
However, if you want a proven, safe CNG car, go for the Tata Punch or Tata Altroz. While their powertrain isn’t as smooth or as good as Maruti’s, these are practical and spacious alternatives.