What sources aren't Legends canon? There's certainly sources from within Legends Canon that say as much.
Actually, yes, the entire premise of the RoT was to eventually produce a Sith strong enough to exact their revenge. That the work of the Banite Sith was cumulative doesn't detract from that, nor does it detract from Sidious being the strongest of the Banite Sith Lords.
One of the sources provided was Force and Destiny, which isn't Legends canon. The other did not establish your position at all.
The premise of the ROT wasn't merely to wait for a single individual, it was to both build power (which including things such as growing a network of spies and developing proficiencies in skills such as assassination and manipulation) and weaken and destabilize the Republic/Jedi (by, for example, putting schemes in place that would pay dividends decades later), and essentially to create the conditions that would enable them to achieve their goal. You're massively simplifying it if you wish to claim that it was just about waiting for a certain individual, and that the work they did solely contributed to making each new generation of Sith more powerful in the Force.
How is Force & Destiny not Legends? It refers to Legends-specific events, to be sure. And here are the quotes:
For a millennium, the Sith maintained the order in secrecy, passing down their evil heritage. As they gained knowledge of the dark side of the Force, their powers increased with each generation.-- Episode 1: The Phantom Menace Scrapbook
Ultimately, Bane's plan produced more powerful Sith Lords with every generation.-- Force & Destiny
For a thousand years we continued to follow Bane's Rule of Two, existing in the shadows, biding our time, growing in power, feeding our hatred. Darth Sidious proved to be the splendid culmination of a thousand years of Sith philosophy and teachings.-- Insider #88: Heritage of the Sith
"Bane's power has been passed down for a thousand years. I vow to be its last recipient."-- Book of Sith: Secrets from the Dark Side
Regarding your second point, those are just things that were to be worked towards by each generation. The ideal final product of the Rule of Two was one Sith Lord who would be powerful enough to return the Sith to power and glory.
Force and Destiny is essentially a campaign setting for a world that’s a composite of Legends Canon and New Canon, and by extension a world that’s distinct from both. Everything from within the sourcebook relates to that unique setting, so it isn’t true with respect to Legends canon.
Regarding the first quote, see my argument with Beni.
Last two quotes appear to be character quotes.