As you may be aware, Navio2 by Emlid is sometimes suggested as an alternate to Pixhawk.
I am no salesman for Navio2 but I am currently playing around with it. Navio2 is a HAT for Raspberry PI that has GPS, IMU, Barometer, PWM control, LED control, and power monitoring built in. It also supports a redundant power supply (power module and UBEC). There is a built-in UART and I2C as well. I have tested the PWM control with BR lights and gripper and they work great.
Its maker, Emlid, has just published a new PI 4 image with updated integration with Ardusub / QGC - not that I use Ardusub.
Because its a HAT, it means that the space currently used for BR Pixhawk is potentially not required and can be used for other electronic payloads. You also have access to the faster RPI 4, and potential inbuilt cooling fans / heat sink case as below. Because of the RPI4 you also get faster CPU processing, USB 3.0 (possibly reduced latency on the USB camera?) and 1GB Ethernet speeds for the tether.
Just an idea…