Finding a product designer that can actually deliver is getting harder nowadays. A junior and even a mid-weight designer without a senior's supervision almost always up the cost of development. Not utilizing the component structures and only fulfilling the needs of the hour setup the project to be refactored during the scale of the product. And it goes without saying that deeply crafted product will set more eyes on your startup and higher chances of getting funded and build audience. I've worked with too many founders to find this out. Your only solution is to hire a senior product designer and pray that they stay or work out. Its expensive, it's risky and often unnecessary. A senior designer can absolutely work fractionally to deliver results to each project. UX being core at each product makes this model even more attractive as the learnings can be transferred from one project to another. Hiring an agency on retainer model is still an expensive business and there is no guarantee that behind the scenes a senior designer is deep diving into your project. Also agencies won't care about long term goals and scalability.