Nice little tree.
Yes, I guess if you let some sacrifice branch grow tall and vigorous, the trunk will thicken eventually.

Ty, So would you say that she is at a good point and should be trained to remain at this height / size?

Do you have any more pics of your mimosa tree? I'd love to see it! I have two mimosa trees atm. One is a freshly germinated seed of a week or so and the other is a couple seasons old. Several years ago I helped my neighbor plant a chocolate mimosa tree and have fallen in love with the species ever since! The blooms are so incredible. I love in Eastern VA so they grow naturally around here its so beautiful driving down the interstate and seeing them!

To me it does not look like the plant has a real.problem. older needles die of, this is normal. And at times small branches die off. As long as the outside of the tree is happy, lush green and growong, it normally is fine.
If this is the extent of the problems, dont worry.

That's awesome news!! I want to wire it up but was worried about adding stress. I feel much more confident now to continue working with it. Thank you so much for your help!

Only wire if the tree is growing at a good speed. Do you have runners in the tree? If not, i would wait. In any case, wiring thicker branches is better done outside the main growong season. They are now so full of moisture that the bark can easily.move against the wood, causing damage.

Oh, OK. So it would be best to wait until after fall to wire? Do most tree species do better with off season wiring? As for this tree at this time there is no runners.