Is working as an intermediary between the bank and the lawyer to take legal action against defaulting clients haram, bearing in mind the lack of knowledge regarding the interest rates?

As long as there is an interest-based benefit (riba), your mediation involves assisting the bank in demanding usurious (riba-based) interest, which is forbidden, regardless of its percentage. Therefore, your work falls under cooperating in sin and aggression. Thus, you are required to stop it, and it is unlawful for you to continue in it, unless you avoid mediating for the bank and any forbidden transaction, and restrict yourself to mediating only in what is permissible.