I think the Ukraine isn't really doing "besiege wei to save zhao", or if they think they are doing they certainly has miss the essential point of it.
The key of sucess of such move is to attack (with high certainty of winning) the critical point of the enemy. This is an open secret, the enemy has to comply regardless. Wei was besieging the capital of Zhao at the time and had very light defense of its own capital. Qi went streight to besiege Wei's capital, Pang Juan had to return and near zero chance of not being defeated because his soldiers were rushing day and night while his enemy rested well and waited in comfort.
For Ukraine to make it work, it has to put a critical location of Russia in risk of imminent fall. No location is within the reach of Ukraine's capability of manpower it has put in. Ukraine can not pull any more troops from existing lines. The two sides are glued to each other at these lines. Russia would closely follow any Ukrainian troops pulling back. Even if Ukraine somehow managed to move all its forces at the new direction without being chased by Russians and took Kursk, it would immediately loose the existing line and Kiev would fall before Kursk, and game over.
The only military value that I can imagine is that if Russia magically gathered a large force in the Kursk region for a supprise attack, Ukraine detected and decided to disrupt such attack pre-emptively. But it isn't the case apparently.
