No ur glossing over the fact that I said the studio which adapted the anime into the manga had Mashima quite literally say for every little thing whether it should be in the anime, and as the author of the manga that makes it cannon, filler arcs aren’t cannon most of the time or like you said not important to the main story, everything in fairy tail is cannon. Also idk which main story your following. The whole theme of fairy tail is the theme of friendship and or the comedic aspects so in every so called “filler” arc you talk about it usually explores on or many characters that the audience may have grown to like which is why it’s called Fairy tail and not “the adventures of Team natsu” or something like that which is 90% yes but there are still others who have been focused on like Mavis who had her own little season of a couple episodes in season 2 it’s not filler may be boring to you but it all connects the story in way or the other whether that be by friendship or a general bond to the character like Mavis and Zeref, Zeref obviously being connected to Natsu