2 circuits for 1 ceiling fan/light combo

I would like to use an existing 12/2 wire circuit to operate the light portion of an outdoor-rated fan/light combo. This circuit is connected to 4 can lights under the same roof, and is operated via a dimmer switch. The idea is to have the single dimmer switch dim the 4 can lights and dim the light portion of the fan/light combo.

The fan/light combo has the typical ground, white, black, and blue wires, with the black/blue supplying power to the fan/light respectively. The white is a common neutral.

Unfortunately, I am unable to pull an additional wire or re-wire with a 12/3 wire through the same wall. I do have the option of pulling wire through other walls. The switch on this wall must be used, however, so I can't simply move everything to a different wall.

I am able to run a wire from under the house from either the same circuit to the fan portion, through a switch on a different wall. I am also able to run a separate circuit (from a different breaker) to a switch on a different wall.

I would like to run a separate hot lead to the fan portion, but the fan only has one internal neutral wire. I am not sure if I *have* to use the same breaker, or if I can safely pull a wire from a different breaker circuit to send power to the fan. My guess is that a multi-breaker setup would be dangerous due to creating the possibility of a looped circuit, not to mention having to know to turn off both breakers prior to servicing the fan.

Again, the stock fan/light has a single neutral wire. I have considered re-wiring, and isolating the neutral of the light and the neutral of the fan. However, they would share a common ground. I'm not sure if this is an issue or not if I ran two different breaker circuits to this fan.

I would like some thoughts and advice. Again, I'm unable to run a 12/3 wire, but must use the switch that I already have wired into the other lights so that all dim at the same time/rate. But, I would like to run a power supply to the fan portion, which will have a separate switch on a different wall. Known options are to splice off a power wire to the fan or pull off of a different breaker circuit that is close by for power (which would use a common neutral and a common ground in the fan, or a common ground and a separate neutral if I can re-wire the fan/light combo to isolate the two).

Thank you for your time on this.