I can only assume that characters like Shatterstar & BTC and such were included in this film because the writers went to the other X-Men movie people and said “Hey, what characters are you absolutely never ever going to use so you won’t care what we do with them?”

Anyways, yeah, you’re exactly right, it’s a Deadpool movie and it follows well in the original’s footsteps. If I have any kind of criticism towards it, it’s that they kind of squandered Negasonic Teenage Warhead after she became the “breakout character” of the original film (at least as far as Internet thinkpiece articles made it seem) so having her really not “in” the film for a large majority of it seemed like a misstep… but then again, like you said, this was kind of like the “Infinity War” of the X-Men franchise, tons of characters but nobody doing much of anything except DP, Cable, Domino, & Colossus.