If you lengthen the trailers tongue, you will also have to move the axle back to keep enough weight on the hitch to prevent fishtailing. Both of these changes will make the trailer less sensitive to directional changes, and easier to back up. 15 - 20 yrs ago I on occasion had to tow an mobile air compressor to remote work sites. The compressors axle to the hitch distance was very short, probably less than 6 feet, so it was very squirrelly while backing it up. Some sites required either backing in or backing out up to a 1/4 mile with no place to turn around. Since my truck had a camper like doghouse on it it was not possible to look out the rear window and I learned that if I could not see the compressor in the side mirrors it was directly behind the truck. If it appeared in either mirror it was time to stop, pull ahead and adjust the path.
Tandem and Triple axle trailers are very resistant to changes in direction at low speeds, but they can be jackknifed with enough determination. If the multi axle trailers that have torsion axles are not set up to provide the proper tongue weight and the trailer frame parallel to the road while hitched up the load won't be equally shared by the axles. Spring axles have an equalizer link between each axle's springs to spread the load
The 10% weight on the tongue is not an old wives tale. While most of the time you can get by with 7 or 8 percent, 10 pretty much reduces the chance of a fishtail getting violent enough to cause loss of control. Either move the axle back, or like in the case of a short utility trailer, place the heaviest part of the load at the front of the trailer. If the tongue weight exceeds what the Hitch's dead weight spec is then a Load Distribution system needs to be installed, which brings in more variables
ok if your boats light you could sleeve the tongue with a bolt on tube otherwise I think welding your extension is best. E trailer probably has exactly what you need.
