Two-sided marketplace at Antler: we built supply first and have zero demand. Did we do it backwards? (I will not promote)

My cofounder and I met at Antler Berlin a few weeks ago, before the cohort started. We're building a marketplace between B2B brands and newsletters: brands tell us who they want to reach, and we plan and run sponsorships across newsletters. We started with supply because it felt easier. Writers want sponsors, and signing them costs them nothing. In about a month we had 27 newsletters across AI, SaaS, and finance, around 670k subscribers combined. Demand is the hard part. We've sent a proposal to one brand and are working warm intros, but nobody has paid us yet. To get the first campaign out, we're now waiving our fee on it and only taking a cut from the writer side. What I'm unsure about: Should we have signed one brand first and then gone and found newsletters for them? The supply feels a bit useless without demand, and writers will lose interest if we don't send them anything. Is waiving the fee on the first campaign smart, or does it attract brands that were never going to pay? For those who've done a marketplace or an accelerator like Antler: what did you have to show on demand before investors took you seriously?