Everyone is entitled to their opinion, but genuinely, if you think that these recent chapters of sxf have been "boring" or "disappointing" or "unnecessary because it has nothing to do with the main characters or major plot" you should stop reading sxf.
This manga, this anime, they are a slow burn. This flashback is part of necessary world-building and exposition. Not everything has to be plot-driven. Not everything has to focus entirely on the main characters. Sometimes to make a good and fleshed out story, to really emphasize the theme of a story, you need to focus on side characters, you need to focus on the world at large. Not everything will be big and bombastic. Not everything should be big and bombastic.
Opinions like yours are a big reason we have rushed 6-8-12 episode seasons of shows as the norm, where we don't get time to know the characters, where we don't get time to let the narrative breathe, where we rush through the main plot. And that format can work sometimes, but most of the time it doesn't.
If you want big and bombastic and action all the time that focuses pretty much on just the main characters and nothing else, go somewhere else. This manga and anime are not for you.