I just started using Joplin (nice, thanks!), so maybe this is somewhere and I simply haven't found it.
Is it possible to change the tab width (looks like default TAB is 4x' '), and make the tab "soft" (i.e. inserting spaces rather than a TAB character).
Though I generally consider myself a calm and rational person, hard tabs in monospaced text somehow drive me insane as it is inconsistent with my other workflows 
No, it’s not possible.
I would recommend an external editor for you. It’s not great for quick small not updates, but if you like things just so it gives you the flexibility to choose a more customizable editor.
I am surprised that this thread hasn't been closed automatically, like so many others. But anyway, in Joplin 1.8.1, in the WYSI editor, hitting the tab key inside a text note doesn't seem to do anything. Can others confirm this ?
Windows desktop: Pressing the Tab key in the wysi editor makes the editor box lose focus so I can't keep writing without moving focus back to the editor box. In markdown mode, it inserts a tab character as expected.
I am not entirely convinced this is the desired behavior, but I am new to Joplin.