Oops would have helped if we showed pictures

CORRECTION.

The prop shaft coupling that I use was a 5mm POWERFLEX (from modelboatbits).

The 5mm prop shaft that I used was a MAXIDRIVE threaded at the propeller end, but plain 5mm shaft at the coupling end.

The ALIGNMENT TOOL that I used was also supplied by modelboatbits. It is the same size as the Powerflex coupling, but turned from a solid bar of alloy and left in place until the motor mounting and propshaft fixings are completed to ensure a perfect alignment.

When the motor and shaft installation is completed, the Alignment Tool was removed and replaced by the identically sized Powerful coupling to ensure as perfect an alignment as it is practically possible to achieve.

I have used the above combination in several of my Classic Model Powerboats that I have restored over the years, and have never had a failure yet.

I run an M5 prop shaft with a Maxi-Power coupling and took some time to make sure that I had a perfect alignment between the shaft and the motor by using a solid coupling when fixing the motor in position.
Any misalignment between the prop shaft and the motor when using this much power will easily destroy most couplings if they are misaligned.

I think my LesRo Rapier is a similar size boat to your Wave Princess (?) and I have been running a similar power motor for the last 3-4 years without any problems.

3500 seems like a high kv for a water model. What does this vessel look like?

I use 650kv for tugboats