Trace width for breaking out 100Ω differential microstrip from 0.5mm pitch BGA
Neck-down traces are a common approach for this issue. If your PCB vendor can deal with the spacing and track width then this then it's a reasonable approach.

Otherwise I suggest using microvias to escape the traces to the bottom layer. Then you have more flexibility in trace width to achieve both Zo(diff) and Zo(single-ended), and the board will have better manufacturing yield.