Hi,
May be this can be considered as a workaround for this.
Since there is no such trigger to trigger when tasks is partially completed, but another approach would be to run a scheduled flow.
The flow should run in regular interval of time, checks the notes and see which requests are completed.
Take this as an example:-
1. Assume the To do list has two tasks as shown below
Now if you see the Notes,i have added Notes like this
Location1- New
Location2- Inprogress
Location3- Completed
where Location1,location2,Location3 represent each request and its corresponding status.
Now let;s see the flow
1. The flow should run in regular interval
2. The flow should list all tasks in to do list
3. Next the flow should loop through each element/task in the list and do the following actions
Next steps will be explained under loop
3.1- use get a to do action to get details of to do task in detail
3.2- use filter array to split the notes and filter out those which are completed.

Expression -
split(outputs('Get_a_to-do_(V3)')?['body/body/content'],decodeUriComponent('%0A'))
3.3- now use if condition to check if the filter array output has elements or not ?
if the condition is True, it means some request are completed so bot should send an email stating this many tasks has been completed
This is to be done in Yes branch, refer this sample
Expression -
length(split(outputs('Get_a_to-do_(V3)')?['body/body/content'],decodeUriComponent('%0A')))
This would send an email like below
Note- This approach needs to be added with one component to make sure it does not send repeated requested all time.
Suppose flow is running every one hour, task A has 4 requests added, out of 2 are completed. user has not completed other 2 tasks in next hour, so when flow runs, bot would message same 2 tasks completion to business which would be annoying, so to resolve that you need to store completion status everytime in excel/sharepoint/dataverse table and then check against it before sending email.
Hope this helps !
Mark it as solution if it resolves your query !