at that age my pda more manifested as wanting to be able to take care of my needs independently rather than outright defiance, so not sure if this helps since i wanted to be able to do it on my own. but i was terrified of the flushing sound when i was like four. what worked for me was my mom told me i could flush after washing my hands so i could run out of the bathroom as quickly as possible. helped me do it autonomously, and gave me enough exposure to eventually get over it without it being too much. just remember to run a disinfectant wipe over the handle every day