Yes, Redmi 6 is a VoLTE phone.
Yes, Redmi 6 can support 2 4G SIM cards at the same time. It is a dual 4G phone. It also has VoLTE support.
Yes, Redmi 6 does have the FM radio support.
If you use the supplied 5W charger, it charges to 18 percent in 30 minutes and 35 percent in an hour.
If your budget absolutely can’t exceed the Rs. 8,000 mark, go with the Redmi 6 without any concerns whatsoever.