meteohub.de

I'd like to be able to log the amount of daylight, to tell if it is a sunny day, clouded or watch the differense in the length of day during the various seasons.

Could this be implemented in meteohub?

Is this easiest archived by modifying a temp sensor like THGR810?

I'm not looking for the solar radiation, just more like a luxmeter..

yes, have a look at the "virtual sensors" chapter in the Meteohub manual. This can be used to map data from a sensor to another type of sensor.