I’m not saying you’re entirely wrong, but you do make some general assumptions that would negate your argument.
You state that an r-selected species would not gain sapience because this has to be taught, but who’s to say their primitive culture didn’t have their offspring live in the wild until they have grown to a point, and then welcome the survivors into the larger group. In a planet as hostile as they had, this would be a great way to weed out the weaker ones and only accept the stronger. When their learning got to the point where they figured out better ways to survive, using technology, more of the children would survive, and eventually they could have grown out of the need to weed out all the weak ones, and simply allowed all their children to survive.
You keep using a single picture to depict an entire culture, a slide showing them holding a Krogan baby swaddled. It’s one picture, and does not tell you how old the baby is, or how many siblings it has. Since we don’t know what Krogan birth looks like, whether it’s by egg or live, we don’t know that this is an actual Krogan newborn, or if it is weeks, or even months, old. The eggs could be much smaller, or the live birth could be more like marsupials, which are live birthed but start out smaller than a finger. The thing is, there is just too much information that is not given to make these kinds of blanket statements.
I agree that it’s highly unlikely a species like this would exist, but I do see it as being possible, simply by looking at our own egosystems. Take a species that has a high birth rate, like pigs, and put them in a place where their natural predators don’t exist, like Australia, and you wind up with a population explosion. Wild pigs are nuisance there, and this is the case for many animals when the ecosystem changes. We killed most of the wolves in the US, and now deer run rampant and their population is mostly unchecked, requiring hunting seasons just to lower their population. In certain places, the counties beg people to just shoot the deer in their yards, because the sheer number of them becomes a hazard for roadways. If they were left unchecked, they would grow and grow until there wasn’t enough food to support them all, and then many would starve as a way of controlling the population.
Just look at humanity. We’ve conquered the wild at this point. Most of our children survive to adulthood, and we no longer have the same threats we once did. Because of this, in a mere two hundred years we went from 1 billion on the planet to 7. We went from 6 Billion to 7 Billion in ten or fifteen years, simply because we learned how to combat some of our biggest threats using technology. It’s not unfeasible to think that an intelligent race who sees other planets as a means to help their population growth would rather go after those planets than allow their population to be controlled by starvation and/or birth control.