Deciding depends on a clear understanding of what your customer wants to “do.”
Historically, designers start from that point of view but don’t generally go beyond what a designer does and into the realm of what product managers are responsible for doing.
I write more about “do” .
Plus what a product manager should do
Only after you have a clear understanding of what your customer wants to “do” and how satisfied they are with their current solution, you can then compare with the list of things a product manager does. For example, a designer usually focuses on how the product is used. But a product manager goes into target markets, positioning, pricing, etc. Things designers typically do not do.