We have multiple tests scripts in Testim and after working for an year using Testim it seems Selenium is better suited for the UI automation and looking for available options to migrate Testim Scripts to selenium.
There isn't any migration tool available. It should still be easier to start with the Testim exported tests and make some modifications than to start writing from scratch.
I've never seen a Testim customer go back to Selenium. Slow authoring of flaky tests is generally a recipe for slowing down your release cycles. You will more than offset any savings in license fees by increasing your selenium personnel costs.