Solar overcapacity is really cheap, because panels are really cheap. Dedicated seasonal energy storage technology doesn't even exist right now, much less economically efficient storage technology. The closest thing would be a massive thermal store used for district heating, but only prototypes are starting right now. Industrial scaling and mass deployment? That's a decade+ away at a minimum.

We could build seasonal overcapacity today, and it would be nearly economically competitive with existing fossil sources.

It's hard to internalize just how cheap solar panels have become. Keeping the same inverter and grid connection, and doubling the panel capacity, nowhere near doubles the cost of the install.