How can I be a better product thinker?

As one who has driven product development, I would say my personal investment has been to adopt a mindset of “taking ownership.”

This can mean many things, though generally for me it has meant thinking things through past requirements. Is what has been requested feature complete? Has configuration, admin, access, and intuitive user experience all been accounted for?

In development, like in building a house, fixtures and finishing touches often take as much time as core design. Having foresight reduces back and fourth, and guides the process before stakeholders are aware of what they will need next.

First of all, disabuse yourself of the notion that people whose official title is "product manager" are any better than you are at "product thinking". I've worked in companies from startup to FAANG, and maybe it's just my dumb luck, or that the organizations as a whole were dysfunctional, but most PMs I've encountered have been shockingly mediocre at executing the core functions of a PM. This is, after all, why engineers like you and me end up being called on so often to handle PM responsibilities that aren't in our official job description.

I would +1 dr_zaius' comment about taking ownership. A big part of product thinking is just literally... thinking about the product as a whole. Caring about the whole thing, not just your piece of it. Ask people why something was done the way it was. Speak up and ask questions and give feedback in meetings, even when it doesn't seem like it's "in your lane". Especially in an early-stage startup, never let yourself think "that's not my job". When I was doing a hybrid eng/product role in a startup, on any given day I might be meeting with upper management to discuss the product roadmap; with legal to discuss patent applications; with sales and professional services to get feedback and feature requests from customers; with accounting to align on the complexities of our pricing model; and of course with engineering to actually guide implementation. Your role may not call for that broad level of involvement, but just remember that your value as a senior engineer or a "product thinker" is enhanced by that flexibility.

Since you've also gotten some specific feedback ("my PRDs could be clearer and better articulated"), obviously act on that. Get more detailed feedback and suggestions about how to do that. Run your PRDs by other people early on -- and from across the spectrum of people that matter, from the business stakeholders to engineers.

Find opportunities to talk to users (or if you haven't launched yet, potential users). Tag along on sales calls. Volunteer to handle customer support rotations. If people throw up obstacles in the way of your doing this, that's a big red flag, and something that needs to be addressed organizationally, or at least squared with the feedback you got to become a better product thinker. Understanding users and their domain is at the core of product thinking, but reading and online research can only go so far. The specific tactics here will differ depending on the type of product and market you're in. A hyperscale consumer web product will be different from a hardware product, and those from a B2B enterprise SaaS product. Understand which type you have and tailor your approach.

Also, check out Ken Norton's blog: https://www.bringthedonuts.com/blog/. He's one of the most highly respected product thinkers in Silicon Valley.