However, all the empire unique things you pointed out are also things that need to be researched or otherwise gotten latter in the game. Certainly in the early game when your source of unity is a monument and the capital building, another planet doubles your unity within months of touch down. Gene clinics, power grids, pleasure domes, all pretty easy to build on new planets as well. Honestly that 30% is probably about the lowest you'll go, not the highest.
This isn't the case, though. The moment you set down, you get the tradition price hike... but you can earn 0 unity until you become a colony a year later, and then you still need 6mo base build time to make a monument/temple/node. You don't get capital unity from a ship shelter unless you have the bonus tradition, so that means you're waiting until you hit pop 5 to get that (plus 12mo to upgrade) - and this also is required for Paradise Domes (12mo to build). Energy Grids (6mo to build) require Planetary Admin plus a tradition that's in another category to give unity. Gene Clinics take tech to build, plus 12mo. If you're counting Art Monuments in the doubling/tripling/etc output, that assumes you have 1500 energy per colony, which is a lot early on, especially if you also want to be a patron to get +20% unity and have resources on hand to buy the Ministry of Culture if it shows up.
All of which is just to say you're definitely not going to double output in a few months. You'll be lucky to double in a few years.
Not to mention the faster research synergizes with this to get you the high-end buildings and +35% unity synths to work them. You can have the high-end buildings by year 40 or 50, at which point you're not going to exceed even just the penalty from founding another colony for years after landfall.
This also assumes no tile blockers create issues... which is also more research that you avoid with only the one planet.
I think this is only hard if you're going all in on influence and making very optimal placements of outposts. Thus the one planet strategy. After all, remember that planets also expand your border, which means even more orbital resources from them, which help a lot at first.
Outposts expand them faster and cheaper. You need focused influence production, but that's not that hard, and this way you don't have to spend time building and flying expensive colony ships to their destinations, and then eating more costs while they're established and building up. You just keep pushing out your borders and digging for anomalies/natives - and if you keep your factions happy, this is affordable to a vulgar degree. As long as you still have the territory, it doesn't matter if it's from a planet or an outpost - the difference is that the outpost will cost 0.5-1 influence and energy per turn, while the planet will be a mineral and/or energy sink for quite a while (buildings, spaceports, tile blockers, upkeep) and can't man the structures built until there's population - and both building them and growing/building pop takes longer than just building more space junk.
Frankly, there should be some population requirement or
something beyond a handful of minerals and a trickle of energy to create, operate, and maintain orbital collectors. It's far too fast and easy to get more from space than planetside, and while I understand why Paradox did it (so "land" has value and it's not all about grabbing "cities"), but it makes Stellaris have some very weird results.
The real advantage of width - even modest width - is resilience. I'm absolutely not convinced it's possible to get reasonable width to match one-wide height for unity or tech, but you pay for that bizarre and slightly nonsensical rapid development by being fragile, and a lot more likely to lose because the RNG didn't like you during galaxy creation.