Hi,
We are extremely conversant with Joomla development and can easily override the core features any way you want while ensuring that no change gets overwritten when Joomla updates itself. We mention this specifically as you have noted that the majority of the work is within the template to improve layout and functionality.
We have developed a large number of custom extensions, plugins, modules and Libraries for Joomla and modified templates with html overrides and Sass implementations to meet specific client requirements.
You have asked us to show our Joomla work.
Most projects, however, that involve a lot of Joomla overrides and customization are subscriber based and require a subscriber to login to experience the features. As such providing a link would be meaningless as you would not be able to view the restricted areas.
If you are interested in considering us for your job, we can send across snapshots of various implementations with explanations of what we have done.
We are very confident that there is nothing about Joomla that we cannot implement.
Hoping to hear further from you.
Best regards