OP in bold and my possible explanations after the bold.

Although we were the first Pax boarding the plane, we were the last ones to receive our welcome drink.

When they offered drinks did they wait until most of the people were there and start in any discernible order? Did they ask the people in front first and then come back row by row?

We were bypassed when they were serving dinner to receive ours last (and we were not allowed to choose between meals!).

Did the FA take your orders before coming out with the trays? Did they go in some sort of order or did they go to the back, then the front, then the side, and then to your row? Are you certain they weren't taking orders in some sort of row to row order?

Did the FA's use a cart? If so was one FA on one side and the other FA on the other? When this happens the pax being taken care of by the slower FA usually suffer. These are usually the pax seated in the middle of where the cart is parked.

Our trash was not picked up (unlike other Pax) until I kindly asked them to take ours.

When you were done with your meal did you put anything on top of the meal like a napkin to signify you were finished? Did you eat slowly and did the FA ask one or more times if you were finished only to have you say you weren't?
Sometimes I come through the cabin to pick up and when I think people are done they become offended that I am trying to take their food from them. After that I don't pick up that persons tray until enough time has passed and I am certain they are completely done.

When I ordered a post-dinner drink (liquor), the FA first started offering this very glass to 3-4 other Pax before actually giving it to me !

On your first 4 segments did the FA's do the dessert service as they went along or did they do a completely separate service? This makes a difference with your order taking. Sometimes I think we are doing it one way and the galley tells me to wait and we will do dessert when we are finished. If they were waiting to do the dessert service and after dinner drinks then I can see that the other rows would be asked before you were because you weren't in the first row.

Also, we were the only ones who did not get custom forms (while others received personal instruction how to fill out the forms), and when I asked for them upon deboarding the plane, we received the forms with a LOT of attitude (and in the wrong language, see below).

Where were you initially when they handed out the forms? Are you sure they handed the forms to the other people? If so, why didn't you ask them prior to the deplaning process? It is possible that they ran out of forms in your language or that the forms that were in your language were in the back of the aircraft at the time you were deplaning and the only forms available were the ones they gave you.

Although one can never be certain about their reasons, we had the feeling that our 'special' treatment may have been due to racial discrimination.

I am sorry you feel this way. My examples are in no way meant to discredit your claims that you were racially descriminated against but to merely point out to you that sometimes things happen for a different reason than you think they did.

If you really feel slighted and that the FA's behaved in an extremely unprofessional manner than by all means write your letter.

I am just sorry to hear that you had four out of 5 great segments and the flight that is preoccupying your thoughts is the bad one. Try not to let the negative experiences overtake your positive ones, especially considering that it was on your honeymoon. Congratulations on your marriage and good luck to you resolving your problems if you choose to write a letter.