Do you play MP at all OP? "Playing wide", to the point of eschewing all internal development other than keeping your conquests passive is the obvious best choice. There are builds you wouldn't believe. The MP community of this game died, in part, because of how obvious it is that playing basically half of this game is all you need to win. The same goes for single player. Going insanely wide is just the most efficient way to play this game. So that does need some adjustment. Simply going FEM/Collectivist + Industrialist/Thrifty and being wildly aggressive is so much more effective than any more passive build (there are variants of that build, but they all have the insane aggression + slavery in common) that there's little point in playing them. I like this game, but I've stopped playing it because single player is so stupidly easy and MP is a race to see who can gobble up and enslave the galaxy with corvettes (and maybe, MAYBE destroyers in a long game) first.
Colonizing 5 planets and then sweeping out to conquer a filled galaxy shouldn't happen. You're right about that. Tall shouldn't be as good as going wide as a strategy, but SOME level of internal development should be necessary, and right now it just isn't.
I don't think this is what the OP is getting at--and I know it's not my concern. In Civ, there was a style of play called ICS--Infinite City Sprawl. It has much in common with the style of play you describe as problematic in Stellaris--grabbing as much territory as possible and eschewing development to the extreme. But I do feel like perhaps when transitioning from describing something the developers focused on in Civ V, "wide empires and tall empires", something is being lost in translation (and indeed, many members of the community had their own ideas about what exactly that meant). I would never put forth the position that the optimal style of play
should be to aggressively pursue expansion to the exclusion of all else, particularly at the expense of infrastructure, since I am a builder at heart. My preferred style of play is to have richly developed worlds (or cities as the case may be from game to game), carefully managed to give my empire a strong economy. I simply like having a
lot of worlds to build in, not just a handful. Heck, when I'm playing Stellaris for fun my typical approach is to hand off my highly developed planets to sectors so that I can develop new additions personally. I wouldn't claim it to be an optimal strategy, it's just what I like doing.
My own objection (I would not presume to speak for the OP on this matter) stems from the use of the terms "wide" and "tall" and the implications they carry with them. Without going on a long-winded rant about Civ, I'll just summarize and say that I was a bit miffed they took a very good economic model in Civ IV which encouraged a measured approach to expansion and neatly curtailed ICS (which was prevalent in Civ III and earlier version of the game), and in the name of creating two styles of play called "wide" and "tall", discarded the old approach the series in a new direction that just didn't really work out. And I've explained why I thought it didn't work out, the short answer being that if more isn't always better then it creates severe problems for an empire-building game.
To use one quick example, it's quite difficult to both make resources a valuable commodity yet encourage a style of play where one eschews claiming said resources in favor of sitting back and building monuments. If a player does not want to colonize a planet, that says that the planet is not valuable to the player, and we need to make sure that's for the right reasons. Generally, my stance on it is that the player should want to expand to that planet, but maybe they're being held back temporarily because claiming the planet could lead to the collapse of their empire's economy (or stability as the case may be). This creates tension in building infrastructure to expand the player's economy before the planet is claimed by a rival. The problem arises when the very act of claiming the planet boosts the player's economy in too short a term, and that is what leads to ICS-style play, which is always a matter to be careful of. This is also reflected in the Stellaris example of conquest + enslavement, where a newly conquered colony becomes productive too quickly.
When I said I think I had Stellaris had the right general idea, I was referring to the drain that colony ships had on energy credits, as 8 credits per turn is quite a large sum early on. It's a nice soft drain that hurts you early on but can be overcome eventually when your economy becomes robust enough to support it. My concern is that it disappears too quickly, and there are of course ways around it (conquering and enslaving as you point out). My preferred approach to the problem as you put it--and I do not deny that there is a problem with rapid expansion at the expense of infrastructure being too prevalent, though I do not feel I've been playing the game long enough to thoroughly detail the specifics--would be to make sure that the ongoing costs of running an undeveloped colony were too high to be sustainable for multiple new settlements early on, whether they were acquired via colonization or conquest. But I would also make sure to keep the costs fixed and not scaling too hard to make sure that an economic "breakthrough" point exists somewhere in the midgame where a strong economy can support rapid expansion. This creates tension in a sort of race to reach that point before your rivals so you can grab more resources and planets. It's also generally the spark for true territorial conflicts, border friction, and the need for a military build-up. While I would never seek to eliminate the early-game rush strategies altogether, I feel it's healthier for the game if the majority of the wars that establish who the big fish in the pond are occur in the midgame, with the lategame conflicts being those that establish galactic dominance. That's my ideal, at any rate. Whether or not it can be achieved or is even, indeed, what the developers wish to achieve remains to be seen.
But that's a lot of text, so here's a
tl;dr. Personally, I'm just spooked by the talk of "wide versus tall" because of the (what I perceive to be) negative effects it had on economic management in Civ V and the complete failure of the game to actually succeed in what it set out to do in creating two distinct play styles. To the perceived problem of rapid expansion being too powerful, I would propose making undeveloped colonies more draining on an empire's economy/stability rather than giving seemingly "arbitrary" bonuses to smaller empires.