Will try not to go into too deep a detail here, as reading other responses it seems a lot of what I experienced has already been said, and you summed up the issues very nicely in your own review, which shows an excellent awareness of where your own development needs to go.
For the story itself, like others, I had difficulty getting into it, but started to enjoy it a lot more the deeper I got. I particularly liked the first person narrative style. It was very personal and direct and really felt like the character was 'speaking' to me. You had a fair number of excellent paragraphs where the character revealed his views on the world around him, such as the opening freedom on the road one, life in prisons, as well as the character description of the german.
In fact, the similes to modern day pop culture were really good. They're risky in some sense, because you require the reader to know them. But this reader at least did, and got a huge smile out of a Johnny Bravo reference!
The biggest issue I had was understanding what was going on. This is likely down to the severe cuts you had to make, and I'd be lying to say that I wouldn't be interested to see what the original 4-5k version looked like, as there were a lot of interesting ideas floated about.
In terms of this story, I think you're right that you overloaded the characters. The german could have easily been cut. I never really got Marcos as the 'villain', and was quite confused as to why he got killed. Likewise, I never got the impression that Carla was killed, only that he faked their deaths in order to escape the military.
I think one of the big problems with the start was that I struggled to get a grasp of what the protagonist was facing. The opening paragraph suggested that he was a wanderer, where in actual fact the story was building to him becoming such, so a transition was sort of missing that informed us that the first paragraph is him speaking in the present, and the following is him speaking about the past.
The prison part as well, I would have probably cut back if not entirely, because I wasn't sure how it related to him being on the road. The close descriptions of life in the cells, while all excellent, goes on for too long in a 1500 word story when you consider that it's all background exposition of how he got into the unit, which isn't so relevant to your core plot of why and how he plans to leave it. Once past that and it became clear he was a 'super soldier' who wanted out the story flowed and the writing picked up to be a whole lot better imo. So in that sense, I think you could have cut all but the first paragraph before the cliff scene, and devoted that backstory exposition word-count to developing the internal crisis the protag has with his department.
Likewise, I think you hit the nail on the head when you said about cutting character count down. I'd have cut the german (a best friend who he doesn't know the name of?) and Marcos, and used their word-space to build up his relationship with Fabio, possibly turning him into some sort of father figure who mentored (maybe harshly at times) our protag into this group. He's the big boss, right? And killing your boss is about as high as you can get to rejecting your current role in life in order to start over. To be honest, you could have even cut Carla, who's only role is to play romantic support. But this ends up boiling down to how much word-space you have leftover once the primary plot is complete, at which point whatever's left can be given to these bonus depth narratives to expand the character's life.
I'll add a few specifics on the writing issues others mentioned. This is aimed at helping development, and it's something that gets worked out with practice, so take it as the help it's intended to be and not as some sort of 'dumping on the ability' as the mind so quickly reacts to it being:
- 'I believe someone
living on the road constantly restart their lives
with one finality' - Here, 'someone' is singular, but 'lives' is plural, so you want to switch one or the other since they are linked to the same topic. So it's either: someone and life, or people and lives.
- prove to themselves that they really changed - This felt weird to me, and it's because the word have is missing after they. So either use 'they've' or 'they have'. They've is probably better, as it's more natural and thus more intimate for your first person narrative, plus it saves one word where every word counts!
- 'you son of a bitch," Captain Fabio's pushes me.' - Two problems here. Firstly, the 's is for possession. So here, it means that pushes belongs to Captain Fabio. That can't possibly be the case, since pushes isn't a physical object like a hat that Captain Fabio can own. So it should be Captain Fabio, or Captain Fabios if the s a legitimate part of his last name. Secondly, when you end speech with a comma, it's only to relay who spoke that speech and how it was delivered. The push is an action unrelated to the speech, even if it is happening at the same time, so you wouldn't link it to the speech with a comma like this. You'd either end the speech with a full stop and have the push sentence on its own, which suggests that the push comes straight after the speech. Or, if you want to link it to the speech as an action happening at the same time, keep the comma, and go: son of a bitch," said/shouted/screamed Captain Fabio, pushing/shoving me.
- 'one more eating tainted, pulled out of trash food' - Something is missing here after tainted, since tainted is an adjective and not an object. The pork can be tainted and you can eat tainted pork, but you can't eat tainted by itself.
- 'shivering in the rough cement coughing with pneumonia in winter. - This was a nice analogy, but you need a 'while' or something after cement, as currently it reads as if the rough cement is coughing with pneumonia which makes no sense.
- 'Snoring and farts causes death - This sentence made me laugh, but something didn't feel right, mainly because of causes. I'd have written this either as: 'Snores and farts cause deaths.' or 'Snoring and farting causes/results in death.'
- 'missing even more pages' - The 'even more' here implies that we're aware pages are missing to begin with. That's not the case, although this could be a fallout of your edits where in a previous version there was mention of books with pages missing.
- 'and walks surrounded and protected by' - As before, something feels missing here, and I'd add the word 'around/about' after walks. Technically, what you've written isn't wrong, but the flow is just slightly out where that extra word smooths out the journey.
- 'who never gets tanned, only burn so much they almost bubble.' - Here, the two halves of the sentence on either side of the comma are so strong they're fighting with each other rather than complementing one another. The back end is a follow on, so I would weaken it slightly to complement with something like: but rather burns so much that they almost bubble.
- gives me hope she listens - This doesn't quite make sense to me. When you hope somebody listens, its sort of a wish that something in the future happens when you speak to them. However, that he's stating that he's still alive suggest that she's either already heard what he's had to say or that she knows what he's going to say. The former is confirmed when in the next paragraph she whispers, "You're right." So the end of that sentence should either reader: 'hope she's listened.' or 'hope she understands.'
Hope some of this helps. I'll end by reiterating my complement for your strong first person narrative. First person is really difficult to pull off well, and grammar blips aside, you did an excellent job of it here, so well done and keep at it!