To determine to tax people, a census is needed. The Empire had done this with the Tax Registry Act
While some villages in the outsiders of nowhere in the Empire might have been able to evade this registry, and even Arial himself had many oppositions to this move that even some nobles in the earlier implementation of this act rebelled against him, by the fourth year of his rule, while he could not say that all the people of the Empire was registered he estimated around forty to fifty percent of the population of the Empire was registered
The reason why Arial did not immediately tackle the problem of taxation in his region is because he did not know who he could tax, who he could collect it from
It would only invite chaos if he forces it too much
This might seem like a small problem but it is not.
Arial since the moment he unifies more than half of the continent under his rule, he had always been thinking of the taxation system