Small custom change to Magento site

General information for the website: e-commerce site
Description of requirements/features: We will have a extension that will restrict what users can see/buy based on there customer group. But our main menu is hard coded, so the if the user is in limited customer group the main Nav will show them categorise that have no products to them.

We are looking for a simple solution to this, possibly just a message in the website header only to the customer group when they login which links to a CMS page (which will have a list of categories hardcoded for them). Or possibly load in a different theme with a different nav menu for the customer group. We could also setup the nav or the cms page to have a rule in it that says, if product count = 0 then hide that category, but I'm not sure how that would work with categories with sub categories.

We are looking for a simple solution that will not add extra loading time to the website ( so good coding practice).
When sending a proposal only send please make sure to explain how you intend to fix this issue, generic proposals will the ignored/deleted

This is a small job but we are ultimately looking for a reliable long term magneto developer and so if this project is successful there could be alot more work from it.
Specific technologies required: magento, nginx, php-fpm
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