

France already has bidirectional power lines to trade electricity with Spain.
They were used to rapidly restore power in northern Spain after the outage and that interconnection actually caused a very short blackout in southwestern France.
The infrastructure France thankfully refused to allow is fossil gas lines, saving Europe decades of gas usage to justify the investment.
Yes, I only listed means to store or produce energy because upgrading powerlines won’t fix power fluctuations : that is due to imbalances between production and consumption, no amount of upgraded to transmission capacity is likely to help.
Load management might help, however. But it’s typically hard to get people to consume more when needed and power shedding is expensive on the electrical operator… Especially since those oscillations were unexpected. Also those things already exist in many European countries.