Properly understand the hesitation, we went through the same thing restoring our Edwardian place when the joiner found this tall narrow sash window with these wonky original proportions that no standard unit would fit without losing the character entirely. Ended up browsing examples of bespoke glasswork  after our architect mentioned they'd worked on similar heritage projects, and seeing actual examples of their slim heritage units (12mm sightlines) in listed buildings gave us confidence they understood the difference between proper restoration and just slapping modern glazing in old frames. They manufactured units that matched the original mouldings exactly while still meeting thermal requirements, which apparently isn't something every glazier can pull off. Took their spec sheet to our conservation officer and got approval first attempt with zero back-and-forth. If you're dealing with unusual proportions in a period property, finding someone who actually shows heritage work in their portfolio beats gambling on someone who just claims they "can do it" without proof.