You could simply reply, "I know that". Works for me :)

Hello! You can say "thx" but it might come off as a little passive aggressive or rude if you're not speaking to a friend you know really well. It's much better to say "thank you" or "I really appreciate that". I hope to have helped you!

Best way would be to answer “thank you” in full

Yes that is fine

Yes, in very informal contexts (like texts messages and social media). I do agree that it could be perceived as cold in some contexts, but that could be exactly what you want in some cases. Context is everything.