My only problem with your view on ''unstoppable'' is that it is incredibly small amount so far. What growth there has been , has been very slow. It has also been mostly bilateral trade with China. So ''unstoppable '' may just mean that if it keeps going , it may reach a meaningful amount by the end of the century.
''I don't know what currency will be used among the BRICS countries.'' There has to be a ''currency'' , or at very least a mechanism for deciding , in real time, what each currency is worth , relative to each other , like the ECU pre Euro.. The other problem is that neither the indian Rupee or the RMB are freely convertible. So the potential for interference , ie what is wrong with US dollar domination ,, is even greater in Brics - Land. The joke with the ''trade in own currency '' you talk about , that kind of '' barter'' , almost certainly used USD relativities to work out how much .
The South East Asian bloc you mention , has the best chance of producing a workable ''Euro'' type system , But they are close in a developmental and business sense, and being part of Brics-land probably doesn't help or hinder that process anyway. They , and S.Africa , for instance , have very little in common [ apart from the Indian diaspora] , and minimal trade.
China and the rest of the world will not change things in any material sense [talking trade currency], until China and India open up their financial systems, and also co-operate with the Eurozone, and produce a true multipolar financial world , Any India/China cooperation is inherently unstable without Europe . I think Xi has stuffed up badly , and probably set any progress on such cooperation back by a generation at least , with his clumsiness over Russia/Ukraine.
For those who feel I am a lickspittle of Uncle Sam with my head in the sand, have a look at the actual Brics trade figures , taking bilateral China trade out. Then ask yourself how long has Brics been going , and how much progress in the trade numbers they have produced.
If you cant be bothered looking , the answer is ''bugger-all''.
Now I have been a fan of a multipolar world for decades , Brics are a sideshow , and probably an impediment.
