Hello Alex,
Welcome to our forum!
There are a few things you need to check. First you would want to ensure the users are linked to employees. You can do this under the user setup, where you have 'Link to Employee /Vendor /Other Name'. This forms the relationship between the User signing in (User table) and the Employee table.
Next, there is a Rep field under Additional Info for customers, here you would want to ensure the correct Reps are selected. Then the last thing is to set up your filter for the customer grid just as you outlined above. The Sales Rep Initial is linked to the Sales Rep ultimately the user / Employee in QuickBooks when you set up the Rep.
Hint, you can set this new filter as a default filter and remove the others if you want Users to ONLY see their customers and no others. If the Reps will be using the Sales Center as well then you would want to ensure only filters with ‘My Opportunity’ or ‘My Activities’ are present, since this is already filtered based on the User signed in. Let me know if this helps.