the UK has done exactly this - to make all Government Digital Services in house. It's been a blockbuster success in my opinion - they have some of the best UX/UI in the world and their websites are ultralight and downgrade well.

There are a few issues with authentication, but there is support. Everything that's related to dealing with the UK government btw is all in ONE website, where everything is explained to you in Plain English - gov.uk

Coming from a country where this doesn't exist, I was blown away by every interaction I've had with the system. It's simply amazing.