A senior researcher embedded in your team, without the overhead of a full-time hire.
A single study answers the question it was scoped for. Most product questions arrive continuously and in smaller pieces, and the ones that arrive between studies tend to get decided without evidence, because commissioning a project for each one is impractical.
When research is run by whoever has time, the method varies with the person, and findings from different studies can't be compared or built on. Consistency is what lets one study's answer inform the next question instead of starting over.
Hiring takes months, and it commits to a level and a specialism before the work has defined either. A standing arrangement gives you the function while the shape of the role becomes clear.
A consistent research function, without the headcount:
Discovery looks at what to build. Evaluation looks at whether what was built works. Both are needed continuously, and which one a given month calls for depends on where the roadmap has got to.
Most of the cost of research sits in the preparation beforehand and the handling afterwards. Having guides, recruitment and analysis already set up is what makes a small question worth researching at all.
Prerequisitecustomer records that can be contacted, with consent in place. Everything else about running participants is part of the retainer.
Context accumulates. Someone who already knows the product, the customers and the previous findings starts each question further along than a vendor briefed from scratch.