We are actively working to improve reference implementation, to make it easier to maintain (easier to read code, type annotations, generally more Pythonic, cleaner design) and use (cleaner documented API, easier to plug in your own implementation of things a content update system might already have an opinionated implementation of -- i.e. the network communication stack).
We hope to build more tools on top of the cleaned up reference implementation once it is feature complete.
For the specification itself, we recently switched to publishing a rich HTML document with cross-linking, syntax highlighting, ToC, etc. https://theupdateframework.github.io/specification/latest/ and added a new section covering some of the repository operations https://theupdateframework.github.io/specification/v1.0.19/#...
I've heard great things about TUF but if you want people to adopt it then it seems like the documentation/tutorials/tooling should be a first class citizen