Berowin directly says in her quote that her secret is practice. She's probably good with Spirit but she's clearly a woman who has spent decades playing with this weave and making it more efficient. It's possible that many different shields are possible. I don't think anyone, including the Forsaken, have taken the time to practice shielding to that degree but they do perhaps know of ways to weave stronger lattices that exceed Aes Sedai shields. We know from Elayne that practicing with Air made her very powerful with it, and Leane/Suian learned to work more efficiently with less saidar. Demandred said that he's had so much time to study war and dueling that no mortal man can match him, so there's a general theme of practice making perfect. It likely extends to the Power too.
Berowin probably found something new... A soft shield that bends. I suspect the standard weave is like a net made of rope and that perhaps some AOL individuals know how to weave shields made of metal wire. This is why Osan'gar says he can get around any shield.... The knot is where the flaw lies.
I think your theory is original and explains many shielding dilemmas in the series.
I suspect that like nearly everything else from the AoL, Shielding probably was something that was taught in a way that made the individual as effective as they could be with the weaves available. Perhaps there were shield weaves that incorporated other (or even all) flows of the Power that allowed for those weaker in Spirit to still be effective at shielding. I could see how earth, air or water could help strengthen a shield and how fire could assist with cutting off someone already holding the source. I always felt like AoL training incorporated assessing an individual's flow strength and weaves were taught that would maximize said individual's effectiveness with any given area of OP use. Thus a man very weak in Spirit might still be able to shield if he as taught a weave that incorporated fire or earth.
IMO this plays into the Aes Sedai losing so much knowledge. Eventually training was made into a "one-size-fits-all" class which eventually lent itself to only those with the strongest potential or an obvious Talent could excel and effectively perform the specific weaves required to become Aes Sedai. Modern Aes Sedai seem to only know one or two weaves for any given effect and those who are weak simply accept that they cannot move forward with these weaves.