It's usually helpful to start with your mandate. Why do you exist as a platform team?
Since it sounds like you're just getting started, your first goals should be in the spirit of crawling (if you've heard the mindset of Crawl -> Walk -> Run). These goals are usually non-coding goals and provide a compass as you mature. They also require you to watch and talk to as many as developers in your organization. (For the purpose of understanding them, but also as a way to get buy-in for your future efforts)
As an example, if you exist to improve the productivity of developers, I would structure goals that provide a foundation and reinforcement of why you exist.
1. Create a baseline of developer productivity
a. Define metrics like "how long does it take to perform common activities?"
b. Measure them
2. Enumerate list of current activities and measure their impact.
a. Are they mission-critical?
b. Are they error-prone?
c. How much time do they take?
d. How many developers perform this activity?
e. How often is this activity performed?
3. Develop a roadmap for improvements (Force-rank improvements based on activities that have a high impact and low effort/risk to resolve)