Totally new and not run on the trailer at all ?

They call that an AZZ KICKER !

And yes, that would cause the OP's issues...for sure.

That's cool but the Bravo3 kit is not available.

Not new, run fine on the trailer first, then drive to launch and fail on start up.

just a fyi,anything above idle rpm on the garden hose can cause the impeller to fail.

Dont ever take anything for granted .... the dealer tech could have Efed up because his IPhone rang or some **** . Ive literally strapped the boat down on the trailer ,and backed into the water almost over the transom , to find air leaks (water) . Pull the inlet hose off your pump and back into water , the water should flow freely out of the hose .
If not, you have a blockage . Ive seen the Hose from the Transom to the Pump collapse , because the outer hose was kinked .

Well, I don't think that is the issue because of the inconsistency and the fact that a fresh impeller picks up water properly when nothing else has changed.
But, enough experienced advice is saying to change that hose, what can I lose, minor expense and some time.

I assume the hose can be changed without removing the drive, would that be correct???

i was under the impression that the merc sea pumps are all the same rear housing and impellor regardless of drive (other than alpha)
and that the front snout/ mount/pulley was different based off the year of production

could be, but I went thru the menu on your link and says B3 product not available

Then why did you dis me...LOL.

It's worth a good shot. Stop the trailer running and see what happens.