Consider offering a portable version of Windows in zip format, which will boot up faster than a single exe.
It can provide both zip and exe format portable versions, each with its own advantages and disadvantages, allowing users to choose by themselves.
Essentially it already is, you can just crack open the portable .exe with something like 7zip and the app is just there in a .7z archive. However I believe if you just run that Joplin.exe directly it will act like an installed version and access your main profile rather than creating and using a new JoplinProfile dir in the location in which the portable app is run - which is the whole point of the portable app.
You're right, it's really installation-free after unzipping. That's it.