Resources for flow mapping
flowmap.blue offers a relatively easy way to create flow maps in a web browser. It's also possible to create the maps from data in a google sheet. https://flowmap.blue/in-browser
Tom Chadwin describes how he created pseudo-3d arched flow lines using QGIS https://medium.com/@tomchadwin/mapping-emotion-fighting-school-closures-with-qgis-28842661bf14
"Design principles for origin-destination flow maps", an article by Bernhard Jenny, et al. discusses all the things you'll want to think about for the cartography of a flow map.