Our company targets both business customers and developers building apps on top of our platform. I’m a non-technical PMM and the first marketing hire in the company. As our marketing team grows, when should we bring a DevRel into the team?

If your company is focused on community building, then I think now is the right time to bring that resource aboard. If you mean Dev PMM or Dev Ex (folks a little more focused on creating resources), you can build this capability internally before dropping someone in:

Ask other technical folks at the company that you think already have a great voice, and have already built trust in the community (founder, solutions architects, pre-sales engineers, product team), to write and present more. Saying "you should write a blog!" will almost never get you what you need on a timeline that you think is appropriate, so you'll have to do a bit of set up.

Consider a prompt format-- ask them to write 50-100 words (expand that if you have someone who wants to build their brand a bit more and is willing to play a bigger role) on why a given feature is useful. Or, why now is the right time for the new product you're launching. Or, why the market is ripe for disruption. Whatever it is, keep the focus extremely narrow. Use those prompts to build out longer blogs, reports, etc. 

Ask the product team to record Loom videos when they're shipping. It's easy for them to do, and gives you another resource for content when you prepare your launch.

Lean on quarterly community surveys to ensure you're capturing top of mind thoughts, ideas, and concerns. Not having someone focused on day-to-day interaction is ok in the short term, but your community has to feel like they have an outlet for being heard and understood.

First off, I don't think there is a template on building out the marketing function, it depends on what makes the most sense for your organization. If I were in your shoes, I’d take the Moneyball/Strengthsfinder approach. You’re already on board as a self described non-technical PMM. If you bring in DevRel earlier, you may be able to cover other non-technical marketing needs by yourself and leverage your DevRel counterpart to help carry developers through the funnel with more technical conversations and how-to. But if your lead funnel is both business customers and developers, DevRel may not help you with your business customers. So if your big gap is top of funnel across both personas and getting leads routed appropriately, you may need Demand Gen. Also, what does your customer journey look like? Do customers start with developer tools from day 1, or do they onboard use your existing software and then graduate to the developer platform after a time/usage threshold has been met? That may also guide what makes sense for you. I’d suggest you start with your present state, think about your goals and your ideal customer journey, address your current gaps, and then decide if the best next role is a more technical person on the team or someone who can help you build demand. Personally, I tend to view storytelling as a key part of product marketing, so I’d probably not lean to a dedicated content resource above those other two as I tend to think PMM could own that function pretty well, especially in a smaller org. 

Ah the famous chicken or the egg problem. DevRel is most powerful when partnered with a marketer who knows how to get eyeballs on the technical content. This is a partnership in the most beautiful form, hard for one to be successful without the other. Since you are the first marketer, it is time for an evangelist. UNLESS you have a developer who absolutely loves writing, and is raising their hand to partner with you on content. But let's face it, when coding calls, your content drops to the backlog so your best bet is to have a dedicated resource. All the marketers in the world won't move the needle if you aren't confident in your message or can't speak the developer language.