There are many synonyms that can be used to replace the phrase "thrash about". For instance, you may use "flail" which means to wave or swing your arms or legs violently. Another option is "wriggle", which refers to twisting movements of the body. You could also use "toss and turn", which refers to the restless movements a person makes while trying to sleep. Additionally, "jerk" is a strong and sudden movement that can be used to replace "thrash about." Other synonyms include "squirm", "twist", "jiggle", and "fidget." These words can all be used interchangeably with "thrash about" to give your language variety and impact.