for Dev software, how do you go about building a PMM team when you are the first hire for the function and that's the first measure of your success?

I might have a slightly controversial take on this one. 

• I think if you are in the business of marketing products to developers, you have to learn the trade a little bit. You must understand developers, developer tools, preferences, contemporary development trends, etc.
• If you are beginning to build a PMM team in a business like this, I recommend bringing someone who is a part marketer and developer. 

Such people exist

• They are building micro-SaaS apps, they are writing books or building content about developers, and they are managing local or online communities around certain topics or technologies. 
• They have a pretty good handle of "marketing through the internet, " basically things like web copy, brand positioning, newsletters, etc. 
• They will find ways to work with Product and Engineering teams to develop differentiated developer-friendly content and "campaigns." 
• They will find ways to get your company on HackerNews.

I know this sounds like too much, but my biggest learning after working for DigitalOcean has been the discovery of this rare breed of "Developer Marketers" who don't even call themselves marketers. They make for excellent PMMs for DevTools, in my opinion.