It later joined the peace process, progress on which has been touted by Aung San Suu Kyi as her priority since she became Myanmar’s de facto leader in 2015.

逗号后为非限制性定语从句修饰the peace process。关系代词which指代the peace process, 在从句中做介词on的宾语，介词短语作从句主语的定语。