Viper Mini is a fine mouse but its on the complete opposite end of the shape spectrum compared to the DA.
The DA is large, ergo and fills out your hand, while the Mini is tiny in comparison, ambi, and optimized for fingertip and claw. Going from 105g to 61g will obviously give you a completely different mouse feel. Personally I love low weight but there are also plenty of people who say they lose mouse control if they go ultralight.

Viper mini does support synapse 3.0 as far as I have tested (I only changed the dpi button to play/pause music), it should save all important settings on the internal memory

The sensor on the mini is perfectly fine and you will never feel a difference to the DA V2 sensor unless you have the fastest arm swipes on the planet. Same case as the Logitech G203 which is supposed to have an inferior sensor than the G Pro Hero but 99% of users can't tell the difference.

I would not recommend going from the DA straight to the viper mini unless you are looking for a mouse that feels much much smaller. Just get the V2 if you are happy with the shape, or maybe consider the normal viper if you want something new with similar dimensions to the DA.