Disclaimer: I'm the founder of Forest Admin (https://www.forestadmin.com) and we provide a SaaS admin panel solution.
3 months at Y Combinator is a short amount of time to build both a customer facing app as well as the admin panel that will support your business at scale. Who would prefer relying on an off-the-shelf (or low code) solution to save time?
Buy vs. build is often presented as a significant dilemma, but most places come up with the same answer - you build software when it is directly related to your unique value prop in the market, and you buy everything else.