I have to create a staff productivity tracker but im not sure how to do it?

I'm interning as a data intern for a procurement department and Im tasked with creating a live dashboard to show how each employee is doing. That means 2 things:

1. looking at how efficient they are at their job

2. looking at the capacity for each employee to receive another case/being overwhelmed with work

So take for example between 2 buyers, Buyer A and Buyer B:

Buyer A received (3x + y) cases

Buyer B received (x + 3y) cases

it is expected to take 3 manhours to complete case X and 10 manhours to complete case Y.

Buyer A will be expected to have spent 3(3) + 10 = 19 manhours

Buyer B will be expected to have spent 3 + 3(10) = 33 manhours

So if each buyer has 8manhours a day, in a week they will have 40manhours.

But if Buyer A took 20 manhours while Buyer B took 25hours to complete the cases,

Buyer A's productivity = 19/20

Buyer B productivity = 33/25

while for that week in terms of work capacity,

Buyer A will have (40-19) manhours to take up another case

Buyer B will have (40-25) manhours to take up another case

In reality there are at least 18 different cases with up to 8 different stages to each of them.

my original idea was to collect historial data on the amount of time they took for each case and find an average on the time spent for each case and their respectice stages, and then also to interview senior buyers to get an idea of how long each case might take. however they mentioned its difficult to say as every case can be considered unique and so the time taken can vary a lot for certain cases if there are multiple stakeholders involve, where some can vary between taking 3 months to a year. So i'm at a loss at how to factor that.

And the key issue is while the Buyers have to update a general excel file to let the company know of their progress (which stage they are at for their respective cases), they dont do it frequently and only do so when management chases them to update it because it isnt their main priority.

My current idea is to have a 'completed' and 'actice' dashboard to observe each buyer's productivity and capacity to take up more or less work

And the workflow i have at the moment is using Power Query to draw Data from the Main excel file and filter info with PivotTable before having the visuals shown on Tableau.