Isn't marketing agencies too obvious/generic a niche — shouldn't you niche down further to differentiate?

Marketing agencies are a great niche — I niche down into marketing agencies myself, I'm not particularly 'different' about it, and I still make more than most people doing an equivalent service. It really depends on how big you want to go: if you want to scale to $100M/year selling to marketing agencies broadly, you'll need to differentiate more. But if you're carving out a smaller niche, you can target whatever you want, especially if you already have relevant experience — that alone can make you stand out. A few good assets to send marketing/creative agencies: full HTML websites, built and hosted one-shot using an HTML template populated with AI based on context about the business.