The "Commands" menu is unremarkable in repo interface.

I don't know what that means, or what exactly you expect, or why it should be more "remarkable" than other items. Please see https://www.mediawiki.org/wiki/How_to_report_a_bug which also asks you to describe what you expect. This is how things look here:

I expect the destination branch to be pre-filled with the default branch name in "Create Change".

That's a separate feature request for upstream.

Make it clear that this will be a draft submission.

What is a "draft"? Why do you think something is a draft? If you refer to the "Work in Progress" flag, I now documented this on https://www.mediawiki.org/wiki/Gerrit/Tutorial

Allow leave blank the commit message to fill in later.

This lacks any explanation why - it's a proposed solution without a problem. Please follow https://www.mediawiki.org/wiki/How_to_report_a_bug .

The blue Reply (comment) button I expect bigger and lower, not at the top, my eyes always miss it.

There is no "Reply" button while creating a draft.

When you haven't clicked the "Publish Changes", the submitted changes are empty

That's why the instructions tell you to click "Publish Changes". It's unclear to me what you expect, please see https://www.mediawiki.org/wiki/How_to_report_a_bug .

This task covers too many different things and is not actionable, so I am closing it.
For future reference, please follow https://www.mediawiki.org/wiki/How_to_report_a_bug .
Furthermore, most aspects seem to refer to upstream user interface issues. It is very unlikely that Wikimedia itself will make any such custom changes and maintain them. You can find upstream's issue tracker at https://bugs.chromium.org/p/gerrit/issues/list where it's more likely that people might be interested in make such changes.