That's a reasonable way to think about it, but is 10 days enough? It seems 12-24 hours would be needed to smooth out diurnal variations, but there is also the seasonal variation of 2x-3x in many places which either requires a large investment in overgeneration or huge amounts of storage. There is also this problem
https://en.wikipedia.org/wiki/Dunkelflaute
which means the storage requirements will be a bit more than you'd think otherwise. You can't get reputable people to quote a price on a whole power grid because of all sorts of uncertainties such as "how many days of outage will people tolerate a year?"
If we use electricity to drive other decarbonization efforts, lets say green steel, or "petrochemical" manufacturing, or sustainable aviation fuels, the grid might become less tolerant of variation rather than more. Use the word "start-up" around a chemical engineer and they're likely to jump out of their skin because starting up a chemical factory is an unprofitable and sometimes dangerous operation. In an oil refinery, for instance, there are systems that produce hydrogen and others that consume it and it reaches a steady state. During startup you may have to make up inputs that aren't available and dispose or store outputs that don't have consumers. There are heat exchangers all over the place to recycle heat but you're going to have to supply steam to some of them and cooling water to others. The system is dynamically stable when it is running but during start-up vulnerable to all sorts of problems, plus people are crawling all over it doing various operations opening up the possibility of human errors such as sucking in storage tanks. In particularly the chemistry used to make jet fuel from syngas or methanol is horribly capital intensive to begin with, increasing that cost 5x by only running the factory 20% of the time takes something that's probably a non-starter to begin with [1]
So far as Voglte a lot of the cost overrun might go away if we just "stayed the course" and built more reactors of the same design. The real sticker cost is probably a bit more than they say it is, but if you could build one bungling free you'd think it could be made for less. It's not just a "western" problem, as the AP1000 is built as a number of "modules" in a factories in China and they waited for years for those factories to figure out how to build the parts and sometimes when they got those parts they were built wrong. If China is succeeding where we are failing it is because they can, politically, raise people's electric bills in the short term in order to dominate an industry in the long term. The main build they are doing now is
https://en.wikipedia.org/wiki/Hualong_One
which is an improved reactor of the kind the French were building back when the French were building large numbers of reactors reliably.
[1] when they really are forced to aviation will probably line up with ground transportation around some single entity fuel like methane or DME