Are people still wondering why Future Trunks is so powerful this arc? His backstory is that he defeated Babadi and Dabura, in the course of that struggle unlocking Super Saiyan 2. He's also gotten numerous Zenkai boosts from being thrashed by Black. Whereas Z's Future Trunks was a neophyte, this Future Trunks is battle tested, trained by the Supreme Kais, and generally speaking has an arsenal attacks almost the same size as Cell's (Masenko, Gallick Gun, Kamehameha, he also picked up the Mafuba almost instantly.) He's a prodigy, which I guess in some sense makes him a Mary Sue but the fact is Trunks has supporting evidence to be a quick learner, to be able to piece together techniques quickly, and to be able to adapt to whatever a situation demands.
Heck, even in Z Trunks knew about the Spirit Bomb (of it, not how to do it) considering he had researched Goku thoroughly, that's why he knew Goku would die of a heart attack and it's why he intervened with the heart medicine. Bottom line is that this Trunks is battle tested, knowledgable, and when his rage breaks he seems to gather energy from around him naturally anyway rather than further transform.
Don't get me wrong, all that energy gathering to enhance Trunks' sword in the finale was in many ways a plot-powered buff, but as far as knowing of / about the spirit bomb and having requisite skill to us it, that should not be an issue for Trunks.