For example desalination, hydrogen production, indoor growing with the light cycle at night or whenever the low demand period is, etc...

For the near future I’d only expect small modular reactors to see much use in areas with unreliable sunlight for chunks of the year. Especially since they could use the waste heat for heating/growing.

This is not true about neither nuclear nor coal.

Nuclear reaction can be slowed down, coal furnace can be used with lower amount of coal at lower temperature.

Nuclear does not need to be turned on or off spontaneously. Nuclear power plant can increase or decrease power smoothly, and that’s practically enough because consumption patterns are easily predicted.

The inflexibility you are describing is nonissue.

High capital cost is. But the largest issue is irrational fears of voters.

Edit: nuclear or coal are not on or off