Currently there is no way to animate trim paths on live text. A User needs to Convert to shapes or masks and go from there. Meaning that any edits to the text become laborious.
Thos animation is a common one too, that I see a lot in titles and motion design.
Adding a "strokes Trim Paths" text animator option would be a great way to implement this, once added you set the trim path speed. then use either the range, wobbly or expression selectors to animate. And honestly I don't know why its not part of AE already. as a text animator it would have great control as the selectors have great flexibility.
C'mon Adobe.