The current behaviour is shared with Tumbleweed 2019-12-27.

After launching (1) installation process (it takes some time) installer
automatically (2) restarts the computer.

The question is what can happen between points (1) and (2)? Number of things:

a) power malfunction
b) installer crash
c) successful installation

So when users sees (2) how can she/he tell if it is because (a), (b), (c)?

Installation is interactive process, interactive by its definition requires
feedback. So please provide one -- a regular summary:

Installation successful
 [ Restart computer ]

And only when the user clicks "restart" reastart the computer.