My issue isn’t with Trunks getting stronger in a short span of time.
Your comparison with FnF Freeza made it sound like that.
I get your problems though. Well, I don't feel there's a limit set for a Saiyajin for him to become a SSJ. I think the answer could lie simply on Gohan and Trunks being different pereonas. Maybe Trunkk has a lower limit set to his base state needed for the chain reaction to trigger and the transformation to commence. I also don't see SSJ as a set multiplier, so I think it's possible that base Trunks could be fairly strong relative to his SSJ transformation. Say base Gohan was at 4,000,000 there, with SSJ teen future Trunks at 3,000,000 and base Trunks at 1,000,000...
It's also worth noting that Vegeta considered Goku as a SSJ when this latter brandished a ~60,000 BP against the Ginyus. Sure, Vegeta knew very little about the SSJs there, but maybe that could be a hint that a 60,000 BP is what's needed to cross the mark between a simple Saiyajin and a SSJ? Could be... if you apply the 50x multiplier, that could leave base teen Trunks and SSJ teen Trunks with justifiable levels.
Sure, that's what I was getting at, I didn't mean to say they were conflicting quotes. Only that Gohan's tutelage seemed to make a hell of a difference on Trunks' improvement... and Trunks training alone would raise the "few months" to "few years". A good argument for training as a duo rather than alone making a heck of a difference.