Storage at nation-state levels is largely solved by pump-storage hydro[1]. Yes, its expensive, especially if you lack natural reservoirs to convert into pump basins, but the economics of it are well known, the technology is mature, and implementations already exist. The real barrier is the chicken and egg problem that until there is urgent need for such storage it won't be built and building it will be time consuming and centralized-cost expensive to do.
> There are limits to how far transmission losses allow you to go.
These limits are higher than you might think. The currently longest HVDC link in Brazil (https://en.wikipedia.org/wiki/Rio_Madeira_HVDC_system) is 2300 km; there is another one being built that will be over 2500 km. For comparison, a quick web search tells me that the "width" of the USA (from the east coast to the west coast) is only 4500 km.
[1] https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...