On plants: Using energy for some/all upkeep would make a weird sorta-sense, yeah! Not really scientific but close enough, and it'd be unique. Sure it overlaps with robots/ME a bit, but they remain biopops and thus very different from robots constructed out of alloys. (Jeez the strategic resource traits for lithoids are just *awful* though, and a natural food production would be kinda weird whether the plants eat food or not)
Interesting about the growth scaling. I had sorta accepted it as a decent trade off for avoiding lag in the late-game... I don't think there's any easy efficiency solution once too many pops are involved. There's a reason I was playing on small galaxies, and now I don't have to.
That said, removing the limiter is interesting because an underlying mood in Stellaris always seemed to be, well, trying to maximize pop growth in the early game then trying to find them habitation in the mid-end game. Hence all my habitat spam, which both boosted growth and accomodated it- at the cost of excessive grind. I also tended to acquire pops through being a lovely nation to immigrate to, being xenophile and egalitarian in most games. Or just conquer worlds and welcome their pops - I don't tend to play pacifist.
The obvious alternative is when one *doesn't* want a xenophilic hodge-podge of species, and is displacing noncitizen pops from conquered worlds. I think that suffered the most from this growth scaling mechanic, since one's precious ubershrooms will completely fail to populate the conquered worlds after a certain point. That's got to be frustrating, while xenophiles and particularly Nihilistic Acquisitioners do relatively fine.
Maybe I'll try another small galaxy with the growth limiting off, and see if I can handle the end-game pop levels. I'm pretty used to spamming habitats, not to mention ring worlds and ecumenopoli. And it's *waaaay* easier than it used to be, since they finally made pops migrate reasonably within one's nations. So instead of tedious reshuffling pops, I can just build the homes and let them come.
I feel like AI nations seem less concerned about living space, content to just freeze planetary growth or whatever. Which I don't mind morally, it's just such a waste in terms of preparing for the endgame. Though maybe I'm just used to establishing my nation's strength by the midgame such that nobody dares try to take my districts.