>And even if the species found a way to “change themselves” to make them not give a fuck, by a virtue of change being possible eventually you’ll get expansionist “strain” again.

For such a species, there would likely be no selection pressure towards such a trait. I suspect it's actually the opposite: being less expansionist could have enough benefits that it would become widespread entirely through random mutations.

Infinite growth isn't possible, desirable, or even "natural".