Regarding bending of a trailer being used at or close to its maximum capacity we have one of these under the 6.5 Scorpion. :ROLLER COASTER 10 | Roller Coaster | Marine | Leisure | Trailer Range
No problem with bending.
Bearings all good after 8 years.
Replaced wheels and tyres at 5 years old as a precaution against ageing. Never had a puncture.
I will admit that the brakes are now rusted out but that's not a single vs twin axle thing.
Tows well - I've never thought " thats unstable, I wish I'd bought a twin axle"
Tow cars have been old Range Rover, old discovery, Volvo XC70 ( max towing weight 1800kg) and currently a Volvo XC90.
Most of our towing is a mile to the slip and back and then a 20 mile round trip for servicing once or twice a year.
When we bought the boat we were supplied with much lighter trailer - similar in appearance to a roller coaster 3 with the single side rails. We towed down to Falmouth and back (from Hamble) with no problems although the trailer frame did have a noticeable bend in it. Changed soon after! I wouldn't advise overloading a trailer but we weren't instantly killed to death the moment we hit 60mph.
In my opinion a correctly specified single axle will be sufficient for your NEEDS.
However, I don't disagree that a twin axle will give some redundancy if you have a puncture. It's up to you to decide what you WANT.
It's fun buying new stuff isn't it :-)