When a client absolutely insists on me using Wordpress for a stupidly bespoke and complicated system that "they will be ultimately selling commercially and just needs to work".

Grow the business balls, tell them that you respectfully decline the offer of works as you feel the scope of works is to unpredictable for the cost/timescale and you can refer them to companies that may take the work on if they wish. They will turn around and either tell you to do what you want for the money, drop the scope of the project for the same price, or double the offered payment.