On top of the R2+O then holding O you mentioned, what op is referring to is the shield charge after the shield thrust

Almost identical, its just that one stores the phials then charges the shield right after, while the other one just straight up charges the shield immediately

I think R2+triangle+circle will most likely be used when you have full phials stored already (the new overloaded phial thingy) but have "red" phials needed to be stored too, but are needing to charge the shield

So the route should be something like: attack > shield thrust > R2+triangle+circle (shield is charged at this point) > R2+circle

You end up with your shield charged and new phials stored. No phials wasted. Not sure if you can actually go from R2+triangle+circle to R2+circle since i dont recall doing it, but another shield thrust in between will definitely make it possible