Tell us what you are trying to do?
Hello, I need some advice on how to set up my toolset website. I have around 30 categories / occupations (such as plumber, home inspector, landscaper, real estate agent...) Eventually there will be thousands of different companies listed. So should I make each occupation a separate custom field, or make one field (such as find a pro) with all the occupations a separate taxonomy?
Is there any documentation that you are following?
No
Is there a similar example that we can see?
No
You should likely make a custom taxonomy for occupation, where plumber, home inspector etc. are terms of this taxonomy.
Then when you create posts (I'm guessing you'll have a custom post type for the company listings) you assign the relevant occupation term to the post.
You could have a custom field for this, but in WordPress taxonomies are much more effecient than custom fields if you later want to use them to filter posts (if you add a search page for the listings that includes an occupation filter, for example).