I tried 3 mods making the night darker. The first two are rather simple and make it pitch black, which is just too much for enjoyment.

This one is much more versatile. The problem though is that oddly, you can only change parameters at start up, not sure why? Why can't it be changed when we want?

This mods changes LUTs in the game. They are technically textures and you can change textures in the game only at startup. I cannot do anything about it.

ah ok, but I thought you proposed several starting parameters, so we can switch from one of your preset to another either?

Could you rephrase that?

I mean, if the mod is installed in an ongoing game, it picks some parameters for the night, right? So you have several preset values, longer night, darker night I think. Why not allow switching between these presets in-game?

Are you asking why you cannot set night length without restarting? My mod works by changing prototypes of some values in the game engine. This is yet another thing that you can only change at startup.
You can find other mods that change the length of day in other ways which may allow to set it dynamically. Just take into account that those other methods will probably affect game mechanics such as production of energy in solar panels.

How is this different from https://mods.factorio.com/mod/DarkNight?

DarkNight is like my mod with the darkest settings and with unchanged duration of the night. You don't have a possibility to set any other configuration there.
Here you have an option to choose colors from multiple presets and to extend duration of the night (only visually).