Running a N channel MOSFET from 3.3V MCU
Just use a MOSFET that has a rated RDson at your drive voltage, NOT the gate threshold voltage (which is when it just barely starts conducting). In this case, even the gate threshold voltage is far above 3.3V. So it seems like you just weren't looking when you picked the MOSFET.

And if you're willing to put that much effort into the method you've dreamed up, then you might as well just go for a gate driver circuit or IC instead.

For slow speed, this is as simple as just having your RPI drive a transistor it can actually drive to pull the gate down, and a pull-up resistor to pull it up. It's basically just placing a second copy of your top circuit at the gate of your top circuit so the transistor pull the gate down and the pull-up resistor pulls it high. Except this time, actually use a transistor your RPI can drive. This would invert the logic so a pin HI turns the MOSFET off.