reports

There's a built-in property called "Marked as completed by" (property definition ID 45), if I recall correctly this is used to track who has already completed the assignment. The users listed in property "Assigned to" (ID 44) who are not listed also in "Marked as completed by" are then still pending perhaps? I don't have a reporting setup to test this right now, but would that make sense based on your reporting data?

Correct, with that "Marked as completed by history" table, I can obtain the time it took for approval by making some extra calculations in the query to eliminate duplicates and obtain the original check-in, as otherwise, with each version change, it updates the time for all approvers.

Also, in the assignment table, when an approval is made, that approver disappears from the assigned to table, and from there, I can obtain the assignees who have not yet approved the assignment.

In the end, it's more complex than one would expect to know this information, but it can be achieved.

Since this seems to be an approval type assignment don't forget to account for users who have rejected an assignment. Their user IDs are collected in the property MFBuiltInPropertyDefRejectedBy (ID 97). From the point of view of the rejecting user their assignment is done and it will disappear from their Assigned To Me view.

The MFBuiltInPropertyDefCompletedBy property will only list those users who have (positively) approved an approval type assignment.

developer.m-files.com/.../

Since this seems to be an approval type assignment don't forget to account for users who have rejected an assignment. Their user IDs are collected in the property MFBuiltInPropertyDefRejectedBy (ID 97). From the point of view of the rejecting user their assignment is done and it will disappear from their Assigned To Me view.

The MFBuiltInPropertyDefCompletedBy property will only list those users who have (positively) approved an approval type assignment.

developer.m-files.com/.../