With regards to your point on electric urns:
What Halachic issues do I need to worry about?
The Federation Beis Din (here in London) provides a wonderful service called "FedTech" which explores all the halachic issues with certain and appliances and have a recommended list of model that are in accordance with halacha.
A full breakdown of the halachic issues can be found here - where it specifically explores the issue of the light going on and off on the urn due to the heating cycle it goes through. The length and frequency of the light going on is dependent on the type of urn one has - either a simmerstat or thermostat. The article explores their use on both Yom Tov and Shabbos.
As far as Shabbos guidance, some pertinent highlights are as follows:
If I take water out from a thermostatically controlled urn, for example, to make a coffee, am I affecting the way in which the urn will heat itself?
...There are two principles of physics which work in tandem and will affect the way the thermostat operates.
Surface area – the larger the surface area of the water, the faster the heat loss, as more of the body of water is exposed to the environment and its ambient temperature.
Insulation – if the body of water is insulated, its rate of heat loss will be affected. The more insulation, the less effect removing water will have on the rate of heat loss. Additionally, the water itself may provide insulation ensuring a slower rate of heat loss.
In an urn, this set of variables will work together in a complex manner to determine the rate of heat loss. Ultimately, making a scientific calculation of the effects of removing hot water from one’s urn is highly complex and effectively unpredictable – certainly not an activity to engage in whilst preparing an early Shabbos morning coffee.
Our own testing has shown that removing water from the urn can slightly speed up the onset of the heating cycle, although others who have investigated these questions have not been able to come to conclusive findings about the effects of removing hot water.
With this as a background, the next question posed is if one is one allowed to remove water from an urn if it is controlled by a thermostat? Poskim such as Dayan Weiss in Minchas Yitzchok, R’ Shlomo Zalman Auerbach in Minchas Shlomo and many others wrote extensively on the topic. Unlike an oven where one should refrain from opening the oven door unless the heating element is already operating, (Opening an oven door when the oven is not heating is problematic as the introduction of cold air to the oven cavity will cause the element to cycle on earlier.) there is room to be lenient (see the article for details why).
As such, the article concludes - as far as an urn controlled by a thermostat:
The generally accepted halachic position on this matter is that one may use an urn on Shabbos even if it is thermostatically controlled.
When purchasing a new urn, it is worth noting that one which has a simmerstat control sidesteps the issues altogether.
Some recommended urns can be found here along with why they can be used and here.