Just wanted to plug the Mordax DATA. When I was starting out I had a lot of difficulty getting envelopes to work correctly. The DATA helped me immensely in that respect. Being able to see what the waveforms were doing was a game changer for me. It does a lot more than just act as oscilloscope - it's tuner function is also tremendously useful when you want to overdub guitar/bass/tuned instruments or jam with others.
That said, there are other eurorack oscilloscopes, but the DATA is the most feature-packed and useful, imho. For a smaller one, there's this: https://www.perfectcircuit.com/dave-jones-designs-o-tool-plus-oscilloscope.html
Inscrumental music for prickly pears.