How can I, as the sole PMM, most effectively lead the product roadmap alongside the product team without getting overly involved in product initiatives and overstepping?

What an undertaking as the sole PMM—how exciting!

Putting myself in your shoes, I'd first start by zooming out and thinking more broadly about what does your ideal PMM + PM working relationship look like overall. Identify your top 3-5 wishes/goals and then meet with your respective counterparts to align on theirs. Then once you have a shared language and understanding, you can review initiatives like a product roadmap and decide together what success looks like. Is it to align on overarching themes/focus areas by target buyers? Is it to go deeper with PMM support on one area in particular, like a new investment in AI? Is it to help bring customer insights and size up different market opportunities?

These are great questions to align with your R&D leaders and once you have that shared mental model, it should help you know where to best step in vs. letting them run their roadmap process as planned.

And for me, I've learned that things shift and change frequently so while it's great to align upfront, be open to flexing as needed to make sure you all are able to best support your business and customers.