Is there a way to change the shape of the system icons on MS launcher, like you can on stock Android?
Yes, you can change the icon packs on the MS Launcher.
it's very common that every launcher provides this feature on their apps.
in case of MS Launcher:-
* Tap and hold on the home screen
* Click on the Launcher Settings
* Click on Personalization
* Scroll down and click on "ICON PACK"
and you are good to go.