The bit that stood out to me was "a Figma file so big it sometimes crashed". I don't know YC but it sounds like this is the kind of thing one of their Mentors would tell them to throw in the bin and take out a paper notebook instead.

In larger companies, it is just accepted to invest the enormous amount of design time into these things but in a startup, anything that hogs your time over selling should be an alarm bell.

It would be different if you were producing a massive UI-dependent product but this sounds like fairly basic sass that doesn't need a Figma design, certainly not initially.