Honestly, that isn't doable at all. Did one-system (building habitats), I was losing a lot of steam by 2300 when I finally got them. Bottleneck being the inability to follow the tech cost progression and, above all, ship maintenance costs in minerals. I'd never have kept up with late-game in spite of fully optimising (IP, Life-Seeded, Materialist).
Well, essentially, for unity, adding a planet is always beneficial but asymptotically approaches a max, because your capital might produce the most unity, but it doesn't produce 5x the unity. This has always been the case, leaving out population only changes things slightly.
The only possible exception was if you loaded up 5 planets with monuments, and have a ministry of culture, and 5 scientists running faith in science, then adding a 6th planet might actually set you back, if it produces less than half the unity of the other planets. But 5 planets was always better than one.
This update may have made unity harder for everyone, but it did not disproportionately affect one planet challenge unity, it was never optimal in the first place. It just made claiming systems uniformly bad for unity, whereas before it had no effect. Adding a
planet is still good, regardless of how many systems you own, if it was good before, but adding systems is always bad, unless you need to add a couple to get your juicy 5th planet.
It changed science a lot more, as previously, claiming systems could boost your science without affecting your costs. Now it does affect your costs. Before this change, OPC, with a science nexus, and using outposts to claim systems, could be king of science. If you had a size-20 homeworld (110% science cost) and added a second size-20 world (+30%) you could potentially reach the point where that 2nd world is giving you less than +27% science, but even thats not easy. Now that claiming systems does matter, and pops on worlds doesn't, one-planet is again not optimal for science.
Edit: was slightly wrong, removing the population component of unity does hurt OPC, just like it hurts it for tech, by reducing the relative effect of planets 2-5 on your unity costs. But, again, 5 planets was optimal before, and still is.