Increasing conversion in SaaS is not only about UX or marketing, but also about data analytics and A/B testing. Most agencies simply charge money for fixed services, but do not provide guarantees on the result. If you are looking for something more reliable, take a look at
https://conversionrate.store/saas-conversion-rate-optimization-services/ . Before starting work, they do an analysis and forecast a real increase in conversion, which they guarantee within a specific time frame. For example, an 11% increase in the transition from a free trial to payment within three months. If they don’t have time, they continue testing for free until they achieve a result. This is a rare approach, so I think it’s worth considering.