I use 10,000# straps, one on each corner, meaning two up front and two in back. They probably outclass the strength of the eyes I’m hooking them to on the trailer, anyway.
As to tongue weight, two things:
1. Quick method is to find point where trailer is balanced, and then move load a fraction of the length forward of that, distance determined by weight of load vs. curb weight of tractor and distance from trailer axles to hitch. This is a rough measure, but useful with changing loads.
2. For anything you’re doing frequently, where you really want an accurate measure, there is a common trick using a bathroom scale. Essentially, a pair of jack stands either side of the tongue, with one just a foot from the tongue and the other 4 ft from the tongue. Jam the bathroom scale under the one 4 ft from the tongue, and a pipe or timber between the jack stands, which the tongue will be lowered onto (after re-zero’ing the scale). Height should be with trailer leveled. Then Archimedes can tell you the tongue weight, based on distances and what is measured on the scale.
You can buy tongue jacks, decent ones are $150-$200, but I’ve measured countless machines with bathroom scales, it’s really not that difficult with some simple middle-school math.