Looking for suggestions on how to travel from South to North Laos

Hi there! Travelling with my partner through South East Asia for the first time, we will go to Cambodia next week to see the area of Angkor Vat and then want to head north (we will finish the trip in Hanoi) and were thinking travelling through Laos. It seems more complicated than we thought though, as it seems that there are not that many busses. Did any of you experience doing that route with public transportation and can share recommendations? Thank you so much!