Suitable coating material for protection of sensors
Various DIY capacitive moisture meters use a PCB for the sensor:



While not directly related to your question I thought I'd mention that you can buy these sensors for several dollars:



Purchasing a readymade kit and relying on the discount of mass production (along with a manufacturer's warranty) is probably easier than designing your own, and etching your own PCB.

I think how you plan to make the probe plays a part in how you plan on coating it; for example you could fit the parts in a test tube, that would meet all the requirements in your list. Such a capacitive sensor would operate similar to the touchscreen of a cellphone.