Hi @Anonymous ,
Do you want to instantly sync updates from Trello to Planner?
I am afraid that there is no out-of-box trigger to achieve this in Power Automate currently.

Maybe you could consider creating a scheduled cloud flow to regularly update all tasks in Planner.
You could try to match the corresponding task by the title and then update it.

last(split(item()?['title'], '-'))
last(split(items('Apply_to_each')?['name'],'-'))
items('Apply_to_each')?['badges/due']
items('Apply_to_each')?['badges/start']
items('Apply_to_each')?['desc']

Result Screenshot:

Best Regards,
Charlie Choi