There are no benefits in joining BRICS. Instead, there are benefits in employing economic policies that characterize BRICS and emerging markets of the Global South, which are variations of the East Asian Miracle that was first started by Japan, then promoted by South Korea and Taiwan, and later Singapore, Malaysia, Thailand, and others, and finally China and Vietnam. That is, infrastructure development needed for mechanized agriculture, mining, and/or manufacturing, heavy coordination between private and public sectors and led by a government that promotes variations of nationalist economics, protection of key industries that have no competition (like utilities) and that have the possibility of impressive development and growth, and export orientation to make industrial sectors grow.
What about the U.S.? As the Council of Foreign Relations:
https://www.cfr.org/backgrounder/dollar-worlds-reserve-currency
and even Stoltenberg of NATO:
https://www.nato.int/cps/en/natohq/opinions_222258.htm
implicitly reveal is that the power of the U.S. and that unipolar global economy lies in the use of the U.S. dollar as a global reserve currency. For that to be maintained, many countries have to be economically weak and thus dependent on the U.S. The U.S. ensures this through a combination of neoliberalism, such as structural adjustment policies via the IMF-WB that encourage client nations to promote free markets, and the military industrial complex, or collusion between rich-controlled industries and the military (with Congress) to use arms sales, military deals, etc., to ensure "freedom and democracy" for recipient countries, or neoconservatism.
These go against the so-called model mentioned earlier, which is said to be a combination of 19th-century Prussian state policies and European mercantilism.
Such a model leads to a multipolar economy where various currencies are used, if not something like variations of special drawing rights, so that no single country can dominate it by being able to have a global reserve currency. Besides, the latter leads to a Triffin dilemma, where one's exports become too expensive for many and importing becomes cheaper, leading eventually to trade deficits and the need to borrow continuously to meet increasing spending.
That's what happened to the U.S., as it began to experience trade deficits starting in the mid-1970s and was able to compensate for that and increased spending by financial deregulation starting in the early 1980s, which in turn lead to increasing debt.