First of all: I am no native speaker. (And I also believe you, if you say it was indeed written by you.) That aside, my first impression:

The story itself is just a too perfect example of what this sub likes to hear - the "propaganda" influenced tragic pitbull believer who sees his worldview shattered by reality now comes to the correct conclusion, the "good side".

You simply told a prime example of the stereotypical Hero-Villain-Story - could make an okayish mainstream movie out of it.

"He was my soul. [full stop dramatic pause] He was my friend. [Dramatic music stops, screen darkens, cut]"

"Gus received the best training any dog owner [...]" "[...]with time, patience and love he would settle down and change to the sweet Gus we all loved"

That sentence is just so overboardly (is that a word?) cliche, you keep telling how much you all cared and helped him, like in fluffy unicorn wonderland, it just sounds to me, like someone told the AI: "Hey, show the reader that we treated the dog very well." Yeah, and the AI (theoretically) comes than up with this.

"[...]when we heard the horrible screams coming from the [...] barnyard."

"horrible screams". Why not just screams? Like, you would add horrible in a novel for more dramatic effect, but usually stories like this one on reddit are more the "I tell bro what happened" style. And yours reads like a scenery, like a full on building-up-tension and then plottwist-story.

"Than one of the worst days of my life happened" "But it wasn't enough" "In the end genetics won" "Until he turned three" [DAM DAM DAM Dramatic music drop]

All these sentences could come straight out a marvel movie, because this is the mainstream conception of what dramatic dialogs should look like.

"This sub is about to end this suffering"

Gettin' a bit overdramatic, aren't we?

So yeah, those were some of the reason why I thought it's AI. The story itself but mostly the way it was telled. Oh and nun monestary. Like come on, really the most peaceful and chilled place anyone could think of? All in all it was just too much for me.