Short term for self-reliance Finland can use natural gas (won't need much), wind and hydro. Since they are a nation with many friends they can also buy electricity from neighbors.

To solve the variable production from solar and wind, most nations should probably have a safety valve in the form of synthesized fuels. Meaning that during summer when energy is abundant and has to be dumped at negative prices, we use the surplus to synthesize fuels instead.

Synthesizing fuel is inefficient, but since you use surplus energy that doesn't matter.

These are options that are viable right now, but there are also promising developments in batteries that could make them viable for season storage too.