MOSFET source measures 12V, still nothing will turn on
First, you should start by drawing the schematic in the "standard" way: from left to right. The first item should be the power supply and then the rest of the components.

Also try to isolate the circuits in small functional parts. I mean, one can suppose that the Wemos and the 7805 will work as expected.

So I would draw the schematic for your questions like this:



Here you will see that the N-MOSFET is not in the lower side so

First problem: You are using an N-MOSFET as a high-side switch

Changing the components, and placing the IRF520 as a low-side switch gives us this circuit



However, the IRF520 if not guaranteed to work with 3.3V of the Wemos so it may or may not turn on. The Vgs of the IRF520 is between 2 and 4 volts, sometimes 2, sometimes 4.

To sum up:


Problem 1 Your IRF520 should be used as low-side switch
Problem 2 You will have to use a buffer for the IRF520 or use another transistor.