Mass Effect Legendary Edition

I had 7400 and it gave me all 3 options so I don't know if other things might factor in, or if it might be a difficulty level thing

Will unlock if you only has 6400 war assett. Did green ending(middle path) on narritive difficulty. Doin my second run for my final achievement to upgrade a weapon. Then it 100% club for me. ^_^

Ah bugger. I picked refuse as it is my preferred ending and so didn't get the achievement for this, Insanity 3 or Long Service.

Thankfully, you can load from the start of the Citadel part of the final mission. Need to redo the last section of the mission but not so bad since it's all just walking and talking after the first few enemies.

i went with the blue ending. as soon as i dropped my gun and touched the 2 small beams, this and all "game completion" achievements popped.
i inmediately pressed the home button, quit mass effect (achievements where still popping) and then i reloaded the game. right before the final choice, so i could change and do the middle/green one

Got to choose synthesis ending with less than 7000 total war assets

So TA says I've completed this achievement but the Xbox refuses to give it to me? What??!

The way the War Assets and Readiness systems have been revamped now, you need a crazy amount of Total War Assets to get the Synthesis ending option. 7800+. It used to be 2800+ in the Extended Cut. So unless you've been doing every single side quest or close, don't be surprised if your only options are the Control and Destroy endings.