Finn was defintely on a suicide mission and he might have damaged the weapon, but not likely. Remember in Independence Day he has weapons to fire. Finn has nothing. He is trying to run into a Locomotive with a Citroen. It isn’t going to do more than scratch it. The angles they show to make it clear how small the speeder is, the parts crumpling from the power, Poe’s earlier comment about how flimsy and decrepit the speeders are (when he puts his foot through the side). It is all story telling to indicate that Finn’s run is hopeless so we are relieved when Rose stops him.
“Never underestimate a Citroen”
I still disagree that the movie is clearly telling us that his suicide run would not disable the cannon. For me it’s not made abundantly clear either way and maybe that’s intentional. As you said about Poe’s decision at the beginning of the film, whether or not Finn damages the cannon is immaterial to the point that the movie is trying to make, so it seems entirely appropriate for it to be kind of ambiguous.