I would not bother with a specific airline card just for the advantages while flying. The OP flies not enough to gain status, plus when he does fly..its business anyway. So he already has all the advantages like lounge access anyway.
There are two reasons why to sign up for a credit card. 1) Signup bonus, and 2) rewards per euro/dollar spent. In the OP's case, the latter is most important.
So I'd look for a card that offers the most per euro spent.
The FB platinum card gives you 1.5 miles per euro. That is 150K miles per year with the spending pattern outlined in the first post. That equals three transatlantic business class promo tickets. Minus the taxes (~300) thats worth about 1500 euro per ticket. So thats a total value of ~4500 euro...if (and only if) the OP can find his favorite destinations in the
promo list.
The regular Amex Platinum card offers lounge access, which is not needed since the OP flies business anyway. It earns 1 point per euro spent. However these points are more flexible, and if Promo award tickets are of no value to the OP...it might be a better choice.
Another random idea could be the Bijenkorf card. The points collected with this card can directly be used in any Bijenkorf store, and is thus very close to cash-back. The rate is 1.25 cents back per euro spent. So 100K euro equals 1250 euro to spend in any Bijenkorf store. Another advantage is that this is a MasterCard card, and therefore has greater acceptance than amex.