MuseScore

This software feels amazing. You can really start composing very quickly, lots of features, good keyboard shortcuts.

GPL, and there's a backing company that makes money with an online and mobile version of it.

Can export to:

Ubuntu 20.04: sound preview worked, first hat that trailing Contra-like sound artifacts (like spd-say), but then it went away?

Feels like a lot of effort was put into usability, including keyboard shortcuts by default, seems like a powerful and easy to start using software to compose music!

Glissando: yes: musescore.org/en/handbook/arpeggios-and-glissandi