Not been there; not done that. But there are three options I can see, of which a collection agency is the least pleasant for all parties. One is to use the small claims court, and accept the loss of the amount that exceeds the maximum. Painful, yes, but perhaps it will cost less than the fee that the collection agency might demand from you. Also, I do not know whether the collection agency will demand to get a fee from you, even if they are not successful in getting any money out of your client. So perhaps you stand a higher risk than you now know. Be sure to read carefully any contract they give you to sign.
Small Claims and Collection Agency are two options. The third option is to file a lawsuit in a regular court. If they don't appear, you automatically win a judgment against them. If they do appear, and if you have a clear case, including evidence of your previous attempts to collect and any offers you had made for payment terms, you will likely also win a judgment against them. That opens the door for you to have their wages garnisheed. Is this hard to do? I don't know. Will the threat of this action bring them to seek a payment deal? I don't know. Would any serious attempt to collect be the straw that breaks their camel's back, and that sends them into bankruptcy court, with the result that you get nothing? I don't know.