Some good advice in here but one note of caution: you mention that you work through different platforms, and that you completed this project "outside of these platforms". It is fairly common for these platforms to require you to agree to terms that prohibit you from working directly with clients that you initially connected with on the platform. If you met this client on a platform like that and then violated your agreement with the platform, then you may have some legal exposure there, possibly including exposure that your client could leverage in a negotiation or legal proceeding.
More generally, this kind of situation demonstrates that these platforms do in fact provide value to the parties involved. So if you're a freelancer and you're considering working directly with clients, be aware of the risk of non-payment.
A real world court would not give a damn about that, because it doesn't have any relevance to the case at all. At worst, OP and his debtors could be kicked from that platform.
From what I've heard, most of those sites side automatically with the customer giving you rights similar to a merchant in a credit card transaction, or really a bit less since that might be all the leverage they actually have and they don't really want to spend their income trying to arbitrate and lose customers who may pay next time.
More generally, this kind of situation demonstrates that these platforms do in fact provide value to the parties involved. So if you're a freelancer and you're considering working directly with clients, be aware of the risk of non-payment.