That’s a nice project !
What’s your actual gearbox reduction ratio ?
Have you considered simple SSS motor with Neugart gearbox instead ? You’ll get more torque with a 56123 420kV and a 1:4 or 1:5 reduction.
Plus you will have following advantages : cheaper, more power (56123 is 15kw capable, useful for takeoff), more torque (around 12-15Nm depending on your reduction gearbox), entire drivetrain can be waterproofed, you can use higher pitches props for higher speeds.
Worth reading for your project 