I have a monorepo of terraform modules and wanted to use something like https://github.com/semantic-release/semantic-release to tag each module if/when it changes. How are people tackling this problem as previously I have used git SHAs but it feels really clunky.

At Previous Job we did exactly that, with our terragrunt mono-repo, except we don't have a monorepo of modules (I wish we did). Each module was its own repo.

yeh the monorepo for modules is really nice as it breeds consistency.

I don't think there is anything specific to terraform. The simplest solution I guess would be to have a github workflow that detects which paths were changed and run semantic-release on each of those using matrix strategy. But we also have modules in separate repos, which makes it much easier with the release process.

We have been experimenting with running orbit - https://github.com/hedlund/orbit as a proxy in front of several GitHub hosted monorepos and using https://github.com/googleapis/release-please to manage per module releases in those repos.