I feel that the third level subclasses make it very hard to work an exciting reason for your expertise. You are already out on your adventure when you pick your subclass for many classes, which means that your backstory is already written. I don’t think it makes sense that a fighter suddenly learns magic out of nowhere or a Druid enters a circle when they are in the middle of the adventure. One could have a little story moment where they choose their subclass, but it feels empty and not recognizing what I believe to be one of the most important moments in a campaign.

Although that makes sense, I think that they choose to specialise I certain areas, so eldritch knight is already trying to get magic, then gets it. Druids could already be part of a circle, just a lowly newbie. So that is how it works. But I think that sometimes a break in the story is needed for something e.g beast master getting their companion. But I don't think it's that strange.