I really like kanbanflow as a product. For example they also have a pretty nice integrated pomodoro timer with interruption logging.

However the reason I moved back to Trello was the interface was a bit old-fashioned, and I found it hard to get my team on-board on kanbanflow wheras it was really easy to get them going on Trello.

I think that's the first Kanban interface I've seen that actually looks enticing. There's something very satisfying about the spacing.

Not open source though (from a quick glance)

I agree that this is important. Optimizing and integrating the tools is a fairly significant part of my work, when I am leading a small team. Webhooks and a plugin framework are decent compromises, but there are so many times when I just want to dive into the code to increment a constant, or subclass and extend a method that doesn't have an externally accessible hook.

You just got them a customer :)