Hi! I started Houkai 3rd during the 1st or 2nd week of its Global release. Houkai 3rd is one of the better mobile games out there that feature weekly content update, smooth gameplay, nice graphics (waifu fo laifu!) and doesn't require heavy grinding. Overall, very F2P-friendly (I'm coming from Pokemon Shuffle so I have a pretty high standard for a F2P game).
To explain the title, I'm not 100% F2P (hence, F2P-ish) since I subscribe to the 60-crystal-daily package. That amount is nothing compared to the whales and doesn't really help you much with the gacha way of this game, so it's mostly to support MiHoYo for such a good game. It's completely feasible you progress in this game decently without spending any money. Casual-ish is because I spend about 2 hours per day for this game (3 in the weekend). I think that's more than a lot of other mobile games which you can just pop out on the bus / train. However, I believe the more time you invest in the