Three options, in order
Configure what exists, integrate what is already licensed, and build only what neither can express. Stated in that order, the question becomes concrete: which specific requirement cannot be met by the platforms already in the estate, and what evidence supports that claim?
Distinguish differentiation from preference
Some processes are genuinely distinctive and are the reason clients choose the firm. Others are simply familiar. Custom construction is justified for the first category and rarely for the second, where adopting a standard workflow is usually cheaper than encoding an old habit.
Budget for the second year
Anything built must be operated, monitored, patched and eventually changed. A build decision that ignores the operating cost is a partial decision. Naming the owner, the support expectation and the change path before construction begins keeps the estate maintainable.