As you yourself pointed out... there is distinct lack of dialogue.
But your writing in 3rd person is actually pretty good. Not unlike Kevin J. Anderson and Karen Traviss's style of writing, that places a lot of info in the persons internal struggles and thoughts. (Whereas Timothy Zahn places a lot of info in the external world. The surroundings.) I like it
You yourself mentioned using flashbacks. Alternatively you could use a sort of interruption.
I don't know what you are gonna do with this character, but one of the few good ways of using flashbacks is the way A. C. Crispin did in Han Solo Trilogy: The Hutt Snare, where she recounted his tales (in The Han Solo Adventures by Brian Daley) as a sort of short stories.
During a period in your stories where your character is making a long travel, they could be used as reminicensing...
I am well aware that I am contradicting myself on another post earlier... But it is a way of doing it.
Coming to dialogue I can only give you a tip. I have issues over that myself.
But what I currently do is actually setting myself in the situation and then write down what I would say and what the opposite end would say given the situation. Pure dialogue, which I then adjust and insert in the story where fitting.
A small audio recorder could also be a great way of making dialogue since you are actually talking and you'd get all the the mannerism of a slightly different grammar due to, it being a spoken language instead of a written language...