You could consider doing a discovery exercise with a number of other departments and agencies to review the information architecture, taxonomies and classification systems in use. All government departments and agencies should have an intranet that is 80% the same. The 20% that is different will likely be due to agency outcomes.
It would be a useful product to produce and publish in the community. Have a reference architecture for an intranet would improve productivity across the public service when people move between departments. They would have a familiar set of information concepts to rely upon.
As for choosing the platform. This should be an Architecture decision based on the departments ICT strategy and future plans for infrastructure. If it is Microsoft based it will be difficult to argue something other than SharePoint, if you are considering adding MS Teams over the top of that then some serious thinking needs to go into the relationship between teams and SharePoint site collections. If the Department is not a Microsoft shop, then the world is probably your oyster. I like Confluence from Atlassian for intranets (it is a wiki) but to be successful you need the right organizational culture. If the intranet is going to be relatively static, you could pick any web content management system you like…Gov.cms / drupal would be the logical choice here.
Happy to help if you want to discuss more.