There are basically three options to deal with overproduction:

• export
• storage
• shutting it done

The cheapest way of using wind and solar actually includes shutting done some of it at excellent days, so the capacity is enough to provide enough power at just okay days. The other problem is that storage is an issue. Right now pumped hydro and batteries are the only ways we have economically somewhat able to actually store electricity economically and both are at the expensive end. So they are usually just used to balance the grid. Hence the optimum is more in having overcapacity. The other option is to use the water reservoir of large hydro river plants, to vary the electricity production. that works rather well. The other big one is exports. As soon as a grid is large enough(continent sized), the weather matters a lot less. So you might see a lot of hvdc lines going from your country to other ones being planned, built and finished.

However most of the world is not even close to that. At 30% and a lot of it hydro, renewable electricity production is mostly just replacing fossil fuel.