I used https://github.com/slaymaker1907/TW5-browser-nativesaver, that still works with the current version 5.3.8, though just in Chromium based browsers. You save the file once and from then on, as long as the tab is open, it autosaves itself.
It's not perfect, though. Paragraphs are rendered by using two br tags, instead of p tags. Link syntax is the reverse of MediaWiki syntax; i.e. [[foo|bar]] links to "foo" in MediaWiki, but "bar" on TiddlyWiki, which trips me up constantly. There's other syntax awkwardness like sensitivity to spacing and newlines. Journals sort in alphabetical, not chronological order.