You can keep the branch commits for detailed history, and look only at merge commits the when you don't need details:

     git log --first-parent -m

This will show a linear history of merges, treating the merged code as being added in the merge commit. You can add -p to see the diffs.

No you commit in feature branches then merge to main branch.

Do you then keep the branch around forever?

The branch history is preserved in the commit graph of the main branch, so you don't need to keep the feature branch.