For the moment, I haven't seen any reason to believe it'll be every expansion. Based on the last two game years, I'd expect one every year.
We got Galakrond's Awakening in the Year of the Dragon and now there is mini set planned for the end of the Year of the Phoenix.
It's good news for players who enjoy the extra content and a mid-expansion meta shake up, but I can certainly understand why many players will get frustrated with 35 cards being locked behind a 20€ / 2,800 gold paywall every year.
My hope would be that, while fun, none of cards in the mini set are too OP or meta-defining.
No, they said it would be "halfway-point of each expansion". Also makes sense with the collectible cosmetic expansion coins, that you get for owning 135 unique cards, which is made a lot easier (that is, possible outside of big cash spending) with the mid-expansion. Considering that those coins get their own section next to portraits and card backs, I expect that we'll get these for every expansion moving forward.