Black is OK here as in most Exchange Ruy Lopez middle games. Just make sure you do not exchange all pieces and you are fine
I don't know how much preparation you are avoiding; 9...Be6 is the line given in The Ruy Lopez: A Guide for Black. I am sure White has seen it before.
Well I didn't know that this move is recommended anywhere I came
up with it, when I was analyzing this position.
I believed that 9 ... Bg4 might give White a more useful then weakening
move. f3 could be a good answer to a possible f5 from Black,
and also f3 gives whites bishop a retreat square on f2.
and maybe it is an idea to place the darksquared bishop
on the h8-a1 diagonal on g7/f6 as Black.
That was the justification given by Johnsen and Johannessen. I have never actually seen the Bishop going to g7 before. You mean you want to play g6, Bg7 and f5 with a blockade on e6 if White pushes the pawn right? I do not think that is a realistic possibility. Let us look at 12...Bf7 13. Nf5 Re8 14. Bg3 which looks ideal for g6 to be played, so 14...g6!? (a) 15. Ne3 h5 16. f3 successfully gains a tempo by not provoking f3 earlier, but I don't know what the advance of Black's kingside pawns actually accomplished 16...Bc5 17.Bf2 f5 18. Ned5 Bxf2+ 19. Kxf2 c6 19. Ne3 f4 (not what White wants to do, but 19...fxe4 20. Nxe4 Ne7 21. Ng5 Bg8 looks bad as well.) 20. Nf1 Ne7 (20...Nf6 21. g3 lets the knight back into the game because g5 now loses a pawn.) 21. Ne2 g5 22. h5 White is better (b) 16. Nd4 Ne7 17. Bh4 (what else? 17.Rd2 Bh6) 17...h5 18. f3 Ng6 19. Nf5 Ne7 not adventurous, but probably best. 20. Nxe7 Bxe7 21. Nd5 Bxd5 22. Rxd5 Rd8 and Black may or may not defend the kingside with a dark square blockade, but he made his own life more difficult that it had to be. 14...Ne7 is probably obligatory.15. Ne3 Nc6 16. Kf1 Bc5 17. Ned5 Ne5 18. h3 and now 18...c6 at a moment when there is no fatal threat to the b6 square, i.e. 19. Na4 cxd5 20. Nxc5 Rd8 21. exd5 Bxd5 and Black is fine. Meijers-Hartl Lienz 2011 had 18...b5 19. a3 Rd8 20. f5 Ng6 and I think Black is in a little bit of trouble; the move a3 jams up Black's plan of ramming White's queenside pawns with ...a3 and White is well on his way to advancing his own kingside pawn majority.
My point is that the position after 9...Be6 is fine for Black theoretically speaking, however I do not accept the explanation that it saves a tempo because f3 is a useful move for White. Black's dark squared Bishop is a bit of a problem piece, it is hard to activate and Black does not really want to trade it off unless it involves changing the pawn structure. 9...Bg4 to provoke weaknesses is a better idea then trying to fianchetto the bishop.
