So in a standard 24 hour day, there is at minimum 6GW of capacity which is always available and always demanded. And for most of year that's 2/3rds of the total demand which ever applies.
The question you are not answering is whether the cost of building a proportionally larger nuclear plant - i.e. one which can meet the upper ends of this scale - is substantially more expensive then a smaller one.
The answer is pretty obviously no: nuclear plants are front-loaded in capital and construction costs, but their relative size has very little impact on the cost of building them, or their maintenance needs, or even fueling costs.
The reality is renewables haven't got anything on that except from an electricity market which mostly doesn't care about them. Start asking renewable generators to well you guaranteed kilowatt-hours throughout the year and watch the "cheap" power skyrocket in cost.
But many components of a nuclear plant have size that scales with power. There are some economies of scale, but they're fairly marginal. It's mostly to amortize fixed operating costs (like personnel) over more output.
I wasn't claiming that nuclear power plants couldn't technically ramp up and down. I will happily stipulate that they could. I was arguing it was economically ludicrous to do so. That's because most of the costs of nuclear are fixed: capital cost, financing costs, fixed manpower costs. If you operate the power plant at low capacity factor, the cost per unit of energy produced increases inversely, just because these fixed costs are being spread over less output.
Nuclear either makes sense for baseload or it doesn't make sense at all. Trying to retreat to an application for which it isn't suited, like covering for intermittent renewables, is a losing game. There are any number of alternatives that would be much cheaper.