A continental spread-out mix (wind, solar, hydro, geothermal, biomass...) reduces production variability ( this is tru even considering a single type of source: https://www.imperial.ac.uk/news/180592/european-cooperation-... )
Overproduced electricity can be stored. V2G will play a major role.
> You are mixing up two things
I doubt so. I was stating that hydrogen may become a clean backup, replacing fossil-fuel existing plants, some of them (burning methane) may be retrofitted. Water electrolysis done in centralized plants gathering electricity overproduced at continental level in order to obtain and locally temporarily store hydrogen, maybe even benefiting from co-generation, will locally burn it (gas turbine or fuel cell) in order to produce electricity when the grid needs it while renewable sources cannot provide enough.
> aren't needed for nuclear because you don't need P2G or any of that, you can just adjust the production
Nuclear can 'modulate' its output up to a certain level and frequency, which in practice are insufficient. Even over-nuclearized France never enjoyed a zero-carbon grid: each year between 6% and 12% of electricity is produced by burning fossil fuel ( https://ourworldindata.org/explorers/energy?Metric=Share+of+... ) because exploiting enough reactors to cope with the mandatory load-following and also with the peak load would be way too expensive.
There are safety-related limits (power modulation proportion, duration of a pause needed after each modulation, modulations frequency...) to nuclear load-following capacity, and the very combustible status is a major parameter. Pertinent document (French ahead!): https://www.sfen.org/rgn/expertise-nucleaire-francaise-suivi...
Proposed translation: "a reactor power output can vary from 100% to 20% in 30 minutes, then after 2 hours can go back to 100% at the same speed, and can cycle this way 2 times per day".
This is quite a good performance when it comes to load-following (French engineers are very good at this), however it is insufficient in the real world (save any ridiculously expensive over-provision of nuclear reactor, most idling) and very weak compared to gas turbines performances.
Those "bandaid" solution are needed with each and every low-carbon type of source, even nuclear. We have to consider the amount of emissions caused, in each type of system, by those solutions.
On a grid when it comes to the "production = consumption" rule there are 2 big types of challenge:
1/ short-term adjustment (under a few seconds). Nuclear, per se, just cannot cope. It is done by the sheer inertia of the turbo-alternator drive shaft. On a renewable system it can also be done this way, thanks to flywheels. It is already done.
2/ long-term adjustments. It is either done by reducing the output of production units, or storing or wasting it or (if production isn't sufficient), by a reserve of "production" units (batteries, green hydrogen turbines...).