Just as a self follow-up here (I hate to do it!), after chatting to Gemini some more more about document creation alternatives, it does seem that Gemini CLI is by far the best way to go, since it's working in similar fashion to Claude Code and making targeted edits (string replacements) to files, rather than regenerating from scratch (unless it has misinterpreted something you said as a request to do that, which would be obvious when it showed you the suggested diff).
Another alternative (not recommended due to potential for "drift") is to use Gemini's Canvas capability where it is working on a document rather than a specification being spread out over Chat, but this document is fully regenerated for every update (unlike Claude's artifacts), so there is potential for it to summarize or drop sections of the document ("drift") rather than just making requested changes. Canvas also doesn't have Artifact's versioning to allow you to go back to undo unwanted drifts/changes.
Another alternative (not recommended due to potential for "drift") is to use Gemini's Canvas capability where it is working on a document rather than a specification being spread out over Chat, but this document is fully regenerated for every update (unlike Claude's artifacts), so there is potential for it to summarize or drop sections of the document ("drift") rather than just making requested changes. Canvas also doesn't have Artifact's versioning to allow you to go back to undo unwanted drifts/changes.