If Marx says that value is determined by socially necessary labour time isn’t that circular or basically the same as the marginalise argument that value is only determined by what is desired by society? I.e. isn’t the “socially necessary” part basically making a demand-side argument? Although at a societal level rather than individual.
Or have the lib brain worms got to me and this isn’t actually the main conflict between LTV and marginalism?
Society's judgment does play a role; it determines whether a thing's exchange value is actually realized, and it affects what the exchange value is through industrialization. But, the exchange value still exists and can be calculated whether or not there's actual demand for a good.
Also, when you say "dedicate a certain amount of labour time," it's important to remember that this is labor per unit; so, if it takes 1 hour of labor to make 10 widgets that's the same as if it took 2 hours to create 20. How much of a thing is produced isn't a (direct) determining factor of the value in exchange.