Even the ridiculous latest plants with 5x cost overruns costing £35 billion seem to have a lifetime electricity cost of around €0.05/kWh. That doesn't seem "really expensive" to me.

Old 70s style nuclear power is under €0.01/kWh.

I also think your 80% is optimistic and it'll start to become much harder to integrate in once we go over 50% total yearly electricity production. Going to need large natural gas plants just lying around idle for most of the year.