Also asked, can you learn with synthesia?
In order to learn with Synthesia, you have to learn the entire thing from memory. Playing along with Synthesia is, as you've found, impossible. Playing the piano as a beginner is hard. I'm fairly sure that those easy songs that you've found are still nowhere near suited for a complete beginner.
Furthermore, what is better than synthesia? The best alternative is Piano From Above, which is both free and Open Source. Other great apps like Synthesia are Musicope (Free, Open Source), Linthesia (Free, Open Source), Rosegarden (Free, Open Source) and Perfect Piano (Free).
Besides, is sheet music easier than synthesia?
Synthesia shows you the notes which you need to play on your screen as if you were looking at a whole keyboard. You don't outright know what to play, it takes some time getting used to, but it's definitely easier than reading the same on sheet music.
Should I buy synthesia?
I wouldn't recommend Synthesia especially since you already can read music. It's a supposed tool to make learning easier but it's doing the opposite by teaching you a different system which is based on reading midi data visually. I would not recommend using it, especially if you want to play classical music.