I actually think this misunderstands the direction in which "Christian nationalists" want to take us. Just like with everything else in the culture wars, they care about Christianity having cultural supremacy far more than any sort of religious mandate. They don't want to, and I don't think they will seek to, establish any sort of mandatory tithing or religious institutions. They don't want to make churches themselves more powerful, or force people back into pews, they just want to chill minority religions and get back to a place where most or all key government officials are culturally Christian and the government recognizes and enforces the cultural primacy of Christianity.