If your motor is the 125hp here: Mariner 125 - Mercury Marine
then the WOT should be 4750-5250.
At 5000 you're right in the middle of the range which suggests with the loading you had that the prop is fine - although you seem to be saying that you could have gone faster so your actual WOT revs might be higher.
Be worth another run with what you would normally have in the boat to see if it stays in the range.
There are various prop calculators on the internet & I'd be inclined to enter your details into a few to see what comes up.
I've just had a play with this one & for 'good all round performance' it comes up with 3 props of 20 or 21 pitch - including what you've got fitted.
ETA you mention 'brutal pick-up'. If that is your engine then the spec listed says that up to 1800 rpm (ish) it only runs on two cylinders so the other two coming in could explain it.