You're right,You are welcome to grow a tree in a pot in any shape or size you wish but there's a bit more than just a leaning trunk to make slanting style.
Your trees looks quite healthy so congrats on good growing skills.
On the design front my eye gets distracted at the first really strong vertical branch/ trunk? I start asking why is the tree slanting when such a strong branch can grow upright? What in the life of this tree has caused that phenomenon an will it continue?
Design means sticking to some sort of theme. Something aesthetically and horticulturally believable.
If the emphasis is more on the slanted trunk and the left leaning branches I think it would be far more pleasing to look at and would tell the story of where it comes from and why far better if that vertical branch is removed or really shortened..
it is not a slanting style yet. I understood that a principle of the slanting style is to grow branches on the opposite direction of the leaning in order to create balance. I wonder if that vertical branch can become more curved with the time (and the weight of the branches) or just a solution is to increase the ramification and foliage to hide that. It is not an ideal solution but also this was a cheap portulacaria afra with several defects