Your points are good, but I think there's another possibility, besides "too big to hide" and "too easy not to invent".
What if there is a very big SSP, but secrecy is extremely important (natsec, earth security, competitive advantage, greed, power), and all kinds of measures at every stage were taken to preserve it? National security oaths, unacknowledged compartmentalized programs, with black budget funding, research done in underground facilities, memory erasure, permanent off-planet postings? To me that is conceivable. You would have two systems, a public system of earth government, and a parallel system devoted to preserving the SSP and off world colonies. I think it could be done. Couple that with a strong PR / propoganda / psyop / disinformation game to discredit leakers, mimic real disclosures in pop culture media, and spread fake narratives, and I think it's like "magic" as in the "art of deception". I think mass psychology could be effectively used to "plug any holes" that the system might have, and over time, with secrecy as an "equal first" priority, I believe they could succeed. Is that not concievable to you in some way, where do you get stuck?
So that's "big and hidden", but not too big to hide.
Secondly, I propose, "inventable but not publishable", as in, what if it's somewhat easy to invent the basic technologies (electrogravitics, zero point energy), but owing to secrecy and economic concerns, these tech have a high "activation energy hump" to get over to actually proliferate in the world? Combine patent suppression, raids on technology to seize and destroy it, intimidation and ensnarement of inventors, and I think you'd be able to keep a lid on it for probably about 100 years, which is what we've seen.
Another way to think about the second point is this, clandestine nuclear devices (dirty bombs or actual fusion/fission) should be possible to deploy for non-state actors, but it's never happened. Surely there's coordinated effort to suppress this tech and prevent it's creation. Now what if SSP tech was an even higher priority, and "approximately equally as difficult to invent / create", if world governments and corporations could prevent a nuclear terrorist, could they not prevent rogue proliferation of zero gravity or free energy tech, especially if they consider that more important?
Again, which part of these arguments is sticking for you? I'm fine if you don't want to engage, I know that even in 2019, there's still a lot of shame and fear attached to these topics, but I see that changing, a lot of whistleblowers, a lot of intelligent, well-researched youtubers (edge of wonder, secureteam10, bright insight), and officially sanctioned evidence (uss nimitz tictac video).
I mean, maybe the whole narrative is just a massive psyop or mass delusion, but there's likely something here that's plausible to you.