I suspect that the "Bell Beaker phenomenon" was a religious one, a mystery cult, carried forth by missionary "priests" who neither married nor bred, an initiation and conversion, in other words, rather than a migration that settled down and interbred.
Possibly, aided by technological know-how and trade.
But how then did Bell Beaker fall into the hands of the highly-reproductive, Steppe-DNA-bearing R1b-312 dynasty? Perhaps, as war-like metallurgists, they were brought in as bodyguards to the priests/traders? Or perhaps they were a branch of the priesthood which threw off its vow of celibacy? (Although I am always doubtful about hypothesising complete reversals of behaviour.)