Rich text fields are a little wonky. ISNULL and ISBLANK don't work
with this field type, period.
To check NULL on this data type field we need to use LEN. It basically calculates the number of characters.
So if you have VR like this: LEN(richText__c) = 0
If you insert an image in the field, the rule will not fire. It will only fire once you add a space ...
The best pattern for this type of validation rule exclusion is to create a Custom Permission, say BypassSubcategoryValidation.
Create a Permission Set and add the Custom Permission to it, but don't permission anything else. Assign the Permission Set to the four users you wish to exempt.
Then, in your validation rule, you can add a final criterion:
Not a direct answer to this question but a suggestion for an alternative approach:
Set up record types for your repair record object
Add a process builder to set the record type based on your picklist values (see this example How to change record type by workflow?)
Add record type based permissions to your user profiles)