For whatever reason, energy production seems to be about meeting demand, rather than finding use for surpluses.

Steel production require massive amount of energy, especially if you want to go coal free. Forest products also depend on cheap energy. An other area is concrete production.

After those I suspect it is food production. Read somewhere that fish farms on land is suffering in particular.

Basically any production dealing with raw materials.

I'm thinking, maybe we get to the point where we have lots of nuclear humming at a constant (ish) rate, and some much cheaper, but unreliable renewables. Can steel mills start and stop? Forestry, ming etc. work on the schedules of wind and sun?

Sounds tricky for sure. Especially because then, renewables will have to compete with marginal cost of nuclear (which is drastically lower than average, since it doesn't include the massive cape, fuel is cheap etc.)

The steel industry here try to build next to hydro, in areas where the price is relative low. The price point can differ by more than 100% when we compare the cheapest regions with the most expensive, for reason that has to do with transmissions. There is also experiments with hydrogen.

A few paper mills have cut down on employees, and in some cases shutdown the plants during periods of low wind.

Food production can't shutdown. A fish farm need to keep the water at a set temperature or the fish dies. Greenhouses that depend on artificial light can't just close down or the plants dies.

I would say that the answer is mostly a no. Short term some industries can adapt a little while waiting for the energy prices to stabilize, but long term it is a hard sell to have employees and building just sitting around.

Mining for materials, waste products, waste heat, noise.

Already have regular extended family complaining about the huge number of noisy shipping containers in their communities. These are middle of nowhere towns.