Very belated answer to my own question as I finally have lengths working! Used this custom attribute expression to display line lengths in a pop-up:
var total_length = 0.0;
for (var i in Geometry($feature).paths[0]) {
if (i != 0) {
var x = Geometry($feature).paths[0].x;
var y = Geometry($feature).paths[0].y;
var prev_x = Geometry($feature).paths[0][i - 1].x;
var prev_y = Geometry($feature).paths[0][i - 1].y;
total_length = total_length + (sqrt(pow(x - prev_x, 2) + pow(y - prev_y, 2)));
}
}
return total_length;