Hi mate,
I've been playing with Los Txuri-Urdin for about half a season and have reason to believe that i've got something right... 'cause i'm currently 3rd in the league. All my players are original players (ie. i didn't sign anyone during the first transfer window) and I am playing a kind of 4-2-3-1. I won't have the tactic with me for the next few days but maybe I can talk you through it for now.
I noticed that in real life they rely a lot on long diagonal balls out the wings. From my experience, the players in FM doesn't quite have the same technicalities required to play that kind of game. Instead I have adapted a short passing game. Your most important players I think are Illaramendi on the DM position as DLP and Javi Ros as anchor. Most of my decent attacks start from these boys whereas Griezmann and Vela up front are almost sure to shoot instead of pass for CCCs.
In terms of positions I actually go for 2 CBs and a right FB in Carlos Martinez. On the left I prefer Jose Angel (De la Bella is utterly useless in this game) and I set him as a WB. In the middle I use 2 DM as mentioned above and a creative force on AMC. I started using Zurutuza but he got injured for 5 months. On the wings I use Griezmann or Vela on the AML position while on the right i use Prieto as an MR. Up front I prefer Agirretxe instead of Ifran.
The basic premise is to use the DM for both defense AND attacking catalysts. This is quite different i notice from real life. Vela tends to be a lot more of a simple offside-beater kind of forward and the central midfielders do not seem to be the orchestrator on the attacks.
If you are going for emulation than I suggest playing a more direct game with focused passing through the sides. It might just work because Prieto and Griezmann are both quite skilful but I personally cannot see it as a consistent tactic, especially in an already winger-centric league.
I don't know if that helps you consider a few things about the team but hopefully you can use my input somehow. I'll try to upload the tactic once i'm back with my other pc.