I don’t use a case, just a screen protector. Been through a lot of screen protectors but the phone has taken a pretty solid beating and still functions like new with exception of battery life. Not sure a case is necessary unless Xiaomi make unusually resilliant phones which I think is unlikely.
Seems like a lot of work to make VoLTE work on your phone. I have a budget Chinese phone from like 5 years ago and all I did was type in a code on the caller part and instantly got VoLTE without any interaction with my carrier (Vodafone) or any downsides at all.