If possible, I'd like a little help with choosing my current sense resistors. From what I understand, the current through that will just be the motor current, so I can use just use P = I2R to get my max power dissipation through said resistor. Is this correct, or is there some complication I'm missing?
If you're putting them in series with the low side MOSFET, you'll only put current through them part of the time. But sizing them to handle the whole motor current rating is probably reasonable (you don't want them to get too hot, after all).
... I'm getting something like a couple of mOhms of resistance that I want tops...
Also thinking of going with 4 termination style resistor for more accurate sensing.
At the very least you need to be careful with layout. You can make a resistor footprint where the main currents connect to the outside of the pads and the sense connections tap off the inside of the component pads. However, you may have 1mOhm or more of resistance in each solder joint (depends on package size), so a 4-wire shut would be more accurate.
An important consideration is offset drift. A basic offset can be calibrated out (e.g. at room temperature), but the drift is unpredictable. Smaller shunt resistor values dissipate less heat but also produce less output voltage and a more susceptible to drift.
Assuming a 5 mOhm shunt (or maybe a bit less - 4.5W is a lot of heat!), you get 5uV/mA into the sense amps. The sense amps are rated for an offset voltage
drift of 10uV/K, which corresponds to 2mA/K. Assuming a 60 Kelvin temperature rise (from 25 deg C to 85 deg C), that's a drift of 120mA. For basic current protection that might be just fine, but for fancy torque control applications that could be a problem.