A couple of problems with the BRICS setting up their own currency. First and foremost, they do not trust each other enough to setup a joint currency. That is a huge amount of trust, because you are handing over control of your currency to other countries.
There is an additional problem that no one else trusts them. Even the Chinese are desperately trying to get their money out of China. You think America freezing Russian assets is bad, Chinese are actually arresting foreigners for even asking simple economic questions.
The Chinese and Indians are willing to pay for imports in their own currency, but only with controlled currency. You cannot use those trade currencies to buy from other countries, like you can with the Dollar. Russia has found out the hard way how bad that is.