Hello again

Balance is a difficult concept.
In the earlier days, people thought it had a hierarchical organization with a lot of neural elements in the subcortical region.

That view has changed now and people like to speak about the dynamic systems framework. This framework sees balance as an interaction between
- the individual;
- the task and
- the environment.

The environment has a great influence on balance, whether it's sitting or standing balance. Walking up stairs will be quite different in the home environment or in a dark cinema room to go to the toilet for instance.
The task relates of course to what you want to do and with increasing difficulty, more demands will be necessary to keep balance. Quiet standing is a basic prerequisite, reactive balance (as a reaction when being pushed) and proactive balance (as an anticipation on an incoming balance challenge) require more balance-related skills.
The individual is of course another crucial part. The motor system with its relevant muscle synergies need to work optimal for good balance; the sensory components for balance include vision, the vestibular system, and the sensory system and then there are of course cognitive aspects like being able to perform dual tasks.

As you can see, one had to look at all these different elements in order consider what treatment would be suitable.

I found it interesting that you mention balance problem of the trunk. Good strength and able to stand...
I would certainly check two things: is the balance problem of the trunk in sitting after coming from supine? Maybe it's dizziness? Then that could mean the vestibular system. Or selective trunk ataxia is also seen in what is called a vermis syndrome where the central part of the cerebellum is affected and what typically gives trunk ataxia in sitting.

Best of luck
Geert