I say populist because it seems he is guided by his conscience and not party fealty, as most republicans seem to be.
I would say conservative republican and not populist. Populists say what they think the average guy is thinking - thus get tagged the term “populist”. Cruz is not that popular. As you say Cruz does what he thinks is right based on his conscience. he is quite conventional and knows and works the system. He is an old school politician and conservative in a party that has devolved into RINOS and establishment hacks.