I think it says something that basic plot points are unclear, like how were they going to destroy the gun. Get really close and shoot it is one possibility. But then I’m not sure how much sense that makes either: let’s fly out on barely functional crafts with limited maneuverability to a bunch of AT-ATs and a big gun and hope it doesn’t fire until we are really close so we can shoot at it. (inb4: it was just like the Death Star trench)
I think it was an act of desperation and Poe didn’t see how futile it was until they were actually doing it. Finn doesn’t have Poe’s experience and we are then shown his craft disintegrating to show that Poe was right and Finn is about to toss his life away on aplan that won’t work. He isn’t even close and he is already in trouble. Rose, again, prevents him from making a stupid mistake. I never once thought he stood a chance but it was in character for Finn to try something like that. TFA had Rey to try to keep him in line and TLJ puts Rose in that role.
I made the point in my first review I think, that this movie is really crammed full of stuff. It’s a dense film.
I enjoyed that aspect, as I was eager to rewatch it several times to suss out the details. But it also is a bad thing because it results in a lot of confusion about what is happening.
Their attack on the cannon was a last ditch effort to buy time till help could come. They didn’t expect to win. Finn might have been the only one who believed that they could destroy the cannon from the inside. I am a bit fuzzy on that right now.
They failed to buy time. Then Luke came and bought them even more time because he is awesome.