The scope should show the response from the ECHO pin (a brief pulse) and a scaled version of the distance. Did you connect the ECHO pin of the module via a resistor divider? What resistor values did you use for that? This wiring diagram was made for a Bela (not BelaMini) but it should be clear how the resistor divider should be wired.
@Tractor could you please try the example linked above (which uses a digital out for the TRIG out instead of analog as you may've been using so far(?)) and see if it works for you?