Given that you have 6 hours, you will have plenty of time to walk to ANY lounge, regardless of whether it is close to the D gates or not!
But given that you are departing on Etihad (which leaves from E), and that you have Etihad Guest Silver which gets you lounge access, why not just use the KLM Crown Lounge?
I, personally, would not pay to use any of the lounges at AMS (there is a new ASPIRE lounge I've not been to, but I don't think it would change my mind) but if you are determined to have lounge access on the way back - I can't help you there.