Hey! Using a catalog ui policy should be the easiest solution. Set the catalog ui policy to apply on the specific catalog item, build your condition (leave empty if it should always apply) and then use an UI policy action where you set the variable from the variable set to Mandatory -> false and Visible -> false. The other attributes can stay with "Leave alone".
This should work in Service Portal as well.
The UI policy action could look like this: (Standard Employee Questions is the variable set in this example)
It shouldn't matter how many Catalog Items are using the variable set. But you have to make sure that the variable set is related to the Catalog Item. Check the "Included in" Related list on the Variable Set record:
Also as I said, make sure there are variables in the set.
Then, once you have added the Catalog Item to the above, you should be able to create a Catalog UI Policy for that Item and in the actions see the variables from the variable set as well as variables only present on the Item
