Honestly, the first product manager for a company is probably not ready to
establish a prioritization framework. The first PM probably needs to focus on
customer discovery, market discovery, MVP intuition, and experimentation. Until
you have established product-market fit with enthusiastic customer demand,
rigorous prioritization is probably bikeshedding.
Once you have that fit, that's when you'll start to get inbound
requests/ideas/complaints from current customers, potential customers, new
market segments you hadn't considered, and sales teams eager to displace
competition. Then you can ...