I finished this over the weekend! I had 92% of secrets initially, then I went back in and polished it off for a clean 100% (a tip for fellow completionists: activating a warp counts towards your secrets tally). I enjoyed it a lot!
The main change to Legacy is the scope: it expands the toolkit with three magic spells, an air dash, and even a few passive abilities for good measure. That's quite a bit more to do (and acquire) than the first game, but it's kept in check by these skills being largely supplementary to the main block-bubble-bolt trifecta— opening new venues rather than outright replacing them. The flipside is that none of these new abilities are explored very thoroughly and have limited use after their initial area. I think that's a bit disappointing, particularly as some of them (like walking on spikes, or slowing time) have a lot of potential for further, more devious use. But that the game doesn't do that sort of gauntlet final dungeon, so I guess that's fair enough.
What I really like, though, are the dungeons. The first is straightforward, but from second-onwards, they each have a mechanic that adds a wrinkle to navigating them: managing water levels, turning on/off power, and flipped gravity. While hardly unheard of, it's an effective way to build off the first game's design, where each dungeon was treated like a big navigational puzzle— now, in addition to the layout, you also have to consider how you can alter that layout to suit your needs. And it becomes very interesting to come back to them later, as new abilities let you sidestep the intended routes and pick up secrets for your troubles.
The only thing I'd ding Legacy for is the writing. Not so much the actual storyline (which is fine, if not particularly exciting) as the dialogue itself, which has the common indie game problem of thinking it's funnier than it really is. Mostly it's just "self-aware": characters snark about the story, crack wise about video game conventions, or wink towards other video games. I will say that it's nowhere near the worst instance of this I've seen— and given how little dialogue there is, it's easy to brush aside— but it's disappointing nonetheless, as Awakening was so earnest, with nary an ironic bone in its body.
In summation, Alwa's Legacy is a really great game and great follow-up. I think I slightly prefer Awakening, but that owes more to matching my particular preferences in game design than to any fault of Legacy. Fans of the first will find plenty to love, here, but I think newcomers will, too. Strongly recommended!