Whether you can still include a headline interest in "business advisory" as the underlying reason why you want to be a paralegal at a commercial law firm depends on how you elaborate on what you mean by that. On the one hand, if you specify that you are interested in directly advising business on strategic decision-making and influencing their operations, I do not think this will work. Even when applying for a position of a trainee/junior associate this kind of justification would probably appear inadequate, as such direct and high-level client advice is normally only given by senior associates and partners.
On the other hand, if you specify that what you mean by that is that you want to constantly learn about business in a variety of industries and what about what drives their decision-making, and that it means you want to be involved in helping business navigate complex matters with huge strategic implications, I think the justification would be appropriate even for a paralegal role. Just like trainees, paralegals also need to understand the context behind the matters they work on, and I constantly hear lawyers explain how invaluable their support can be on time-sensitive and complicated matters.
The slight shift in the language and focus of these two formulation of what business advisory means may not appear very significant; however, I think for this kind of question it is crucial to showcase a nuanced understanding of the different types of work and responsibilities that exist for the different roles in a commercial law firm. This takes me to my final point: to really demonstrate your interest in a paralegal role specifically, I would advise you to also mention and explain your interest in the typical tasks a paralegal (as opposed to a trainee/junior associate) gets to work on
