Nuclear power isn't bad if it's a high-temp reactor you have an alternate use for the heat (water desalinzation, or motor fuel, ammonia, or hydrogen synthesis). Additionally newer reactors should be capital costs down as you caputre some of an economy of scale. Additionally is some systems you could mix natural or bio gas in the same plant to adapt to demand. (and you can be paid for kW of variable production capacity as well as purely on a kW basis
Besides quick-start systems tend to be much less fuel-effecient than always on systems even with the same fuel (Oil and Gas)
My though is a nuclear base load and complementary renewables The sort of reall good grid (Read really really really good grid) needed to balance renewables is going to be really really expensive, and in most countries cross-national (read poor energy security)
Converting coal to biomass doesn't make the plant any more adaptable in terms of startup/shutdown times.
Really the only practical option is distributed and smart storage or quick-start oil/gas. (maybe biogas if you can find enough) (Even though I think nuclear is really great as a base load, it doesn't much address the variability problem, as best some reactor plant designs allow a way to add some variable capicity in combination with a gas or liquid fuel