you dont need necessarily a 4 later for the job.
But even if you do you can get 10 peaces for 40$ in pcbway.com for example. Design and circuit is not complex at all but cooling the heat through copper area will take More space. There is a new Technologie like heat pipes that can also absorve the heat for high speed processors.l in the pcb trought via holes.
The circuit is simple:
DRV8323-> 4,2€ (60V Three-Phase Smart Gate Driver With Three Current Shunt Amplifiers)
CSD88584Q5DC->4,3€ x 9(at least 6 but 9 will make less heat dissipation, because the current will be divided in 3 per phase, the more the better)
bq78z100-> 4.1€(battery gauge up to 2 cells)
bq24725A-> 3,2€(battery charger, only need external power supply like 24V)
RF can be:
CC6540R2 is a 2 Core processor, one with cortex M3 48MHz MCU and the other is a cortex M0 with Bluetooth 5 long range connected to RF 2.4Ghz extender CC2592 with 22dBm more TX power.
Bluetooth 5 can go up to 1.5km because receiver is sensitiver.
Or a simple sub 1GHz RF:
MSP432 Arm Cortex M4 48MHz with CC1120 RF transceiver with -120dBM Receiver sensitivity.
Because the device will be under water, the RF in controller has to have higher TX power
and the RF on the motor has to more sensitiver. If you only want to control the motor but if you want to get battery status, the TX and RX will be on both devices the same.
Also choosing the right antenna will make a big improvement.
So that the mosfets don’t get over heated or burned out, the DRV8323 has an current protection watcher that can be programmed and once it’s over 160A it gives MCU the notification.
If the current is 160A and the time gets over 10s for example, the MCU goes to safety mode and waits until mosfets cool down (allowing low speed only).