For your point (1) I can offer you https://sequence-diagrams.netlify.app which is my years-old quick diagramming tool for Mermaid and works similarly.
For your point (2) I offer the same alternative, but with the caveat that of the 3 criteria of elegant/thoughtful design, beautiful diagrams, and export functionality, it meets only the last 2 (and I claim little personal credit in any case).
> But I'm not aware of a reason why anyone wouldn't want to use swimlanes.io
Totally fair; personal preference. For me I could quite easily replace swimlanes in your comment with my own site, and I'm sure others would have alternatives that also seem an obvious first pick - there's rarely one size fits all.
