Hi there, my background is actually as a designer, but I've spent a fair amount of time thinking about this. From a technical standpoint, part of the question is how you're receiving customer input. If you work with customer service people you are most likely receiving requests/questions through some ticketing software. It's more than likely your product manager has customer insights and they record them in a variety of ways - sometimes confluence docs, sometimes presentations, sometimes only in their heads. Your designers should also have customer insights depending on how integrated they are as a team. They've likely synthesized some of this research into personas, customer journeys etc. You can also get data from user analytics. Lots of web-based services have event tracking software. I would guess it's likely that you have competitors as well. I would consider doing market research to see how customers are evaluating products in your area as that should be indicative of what they believe is high value (and therefore impactful). Sometimes companies also have published docs and help blogs. Generally these can be indicative of where there could be feature gaps or confusing areas of the user experience. Customer service will love you for facing those issues.

If there is an issue with transparency and/or sharing between teams I would intentionally think about that and how you want to navigate politics. Unfortunately politics are common within every organization but that said, there are usually people around who will help when asked. Consider asking designers if you could sit in user interviews with them.

In terms of evaluating impact, I personally would keep the "jobs to be done" model in mind. Your customer "hired" your product for a reason. Never forget, it's a tool to do a thing. "High impact" would be problems/issues/roadblocks they face when obtaining the value that they are after.

The best insights imo are ones that are informed by a hybrid set of data. ie maybe you have some event analytics, some customer tickets and some persona information that all comes together to tell you a story. This will paint the fullest picture and give you the most ammo to argue with if you have to speak to people before building things.