Try to understand why your move is inferior to the correct one!
You seem to be assuming that a non-book move will automatically be inferior to the book move.
Try this for a laugh... a book line in the Petroff.
This line had been played thousands of times, over several decades. The three book moves were Bd2, Bf4 or Re5, and Black typically gained the advantage against any of the three moves. The statistics for this position were : 35.7% White wins, 21.4% Draws, 42.9% Black wins.
Advantage: Black.
Except that after the non-Book move 14. Bh6!! Black can resign.
Well, why black do ...... cxd5?
Doesnt he see 2 move tactics? Like, cxd5, Rxe6( open diagonal).
Why not O-O-O then?
In general if you dont see 2 moves tactics, u will never go beyond 1200- 1300.
... and one variation of the marshall attack in the ruy lopez. …
Not sure that is a good choice if you would rather avoid the necessity to learn many variations. However, if you like the idea of replying to 1 e4 with 1...e5, you might like to play over some of the games in First Steps: 1 e4 e5.
https://www.newinchess.com/media/wysiwyg/product_pdf/7790.pdf