In my experience a prioritization framework is foundational to establishing a great working relationship within your own team and stakeholders. I'd also argue that if executed well in the beginning, the framework may not change much regardless if you are the first or 10th PM at a company. In fact, it may be a bit more straightforward as a solo PM since the prioritized list of needs and deliverables is a direct negotiation between you, stakeholders and your delivery team(s). As the product organization grows you'll notice that blockers, dependencies and enablers exist within your own product peer group as much as your business and engineering partners. Product teams even become each others' stakeholders along the way.
As the first PM, you should always establish ways of working and a framework for how the prioritization process will run: how you collect and understand requirements, validate impact, align on definitions of done, agree on and then communicate a prioritized delivery plan. I've used different versions of the RICE and MoSCOW methods in the past, which I suggest you look into.
Don't skip out on doing things in a documented and structured way because you are riding solo, trust me it'll pay off in the end :)