This does look like a good solution. The other similar solution Tidly that I’ve run used Google’s App Engine (which is a bit weird and old).
There are some other open source Firebase competitors like Supabase https://supabase.io/ that could fit here.
But, to the task at hand for @stobot – a multi user, online hosted TW with everything you need would be a magical thing for many communities and teams. Thanks for bringing this up!