First of all, I LOVED the concept. It was very unique and interesting, having this little sandbox filled to the brim with existentialism and even touching concepts of the self and loneliness. Plus, being able to place the cards wherever you want made it feel like some sort of sims like game or even one of those "Create your own adventure" type of books, which also gives it some appreciated replay value. A really nice touch was the final summary of your journey, as if it was, indeed, a book. Fantastic idea.
The simple yet wholesome design, as if it was hand-drawn over a white paper, reinforces the concept of "vast nothingness" and I really enjoyed that.
I had two tiny problems though. Maybe it's because I played the browser version, but my audio only came from the left ear (or at least it was very low on the right one, I don't know) and I got a soft lock in my second run because I only had personal cards to choose and I had no cosmic card to put them on (that black hole is super buffed, so I'd suggest maybe giving it some sort of mechanic to prevent soft locks like stopping it from eating the last cosmic card that accepts the personal cards you have in your deck).
Great experience over all ^^