There’s obviously a chance for a big break through as the first defences are always the heaviest and it should get easier after that

That’s the opposite of Russia’s defensive doctrine, which would only be exacerbated further by the ability to deploy reserves where necessary. It could be a different story if Ukraine wasn’t having to commit their exploitation forces just to try to reach the first lines of defense; even if they break through tomorrow they won’t really have the manpower to do anything with it.