Why was it so easy to defeat Toneri, Kinshiki and Momoshiki?
Naruto and Sasuke during Boruto saga are way stronger than the version in the war arc, so any actual threat that is Kaguya or even just slightly above her is not gonna do much against them and did so because they were nerfed in the movie. 

As for how Boruto's rasengan finished the enemy is because Naruto used his chakra to charge it and you can expect that his rasengan is going to be over powered. 

These are reasons I believe so, as you can't expect them to be same level when they fought Kaguya as they did not have experience with their power and it's already shown in this series and many other that you become more powerful as you grow older until certain age.