Subcontractors face the biggest payment risk at project end, when clients shift from goodwill to contract language. Document every variation in small pieces as you go. That paper trail is your leverage when the client tries to withhold money.
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Subcontractors struggle the most at the end of the project because at the end of the project, the goodwill often disappears and then contracts suddenly appear to matter again. So your corresponding PM from the client has been like, don't worry about the contract, just get the work done, we'll sort it all out later. Then we get to the point where we're sorting it all out later and like, no, we're never paying you that. That's what happens in construction and that's not how you want it to happen. You want to have all of the evidence behind you because ultimately whoever has the money, and this scenario the client has the money and they have to pay you, has the power. And if you have slowly but surely been getting all of these variations throughout the project in smaller chunks, now you've got the money and they might try and get it back off you, but you have the power because you already have the money.