Kanban might also be something to strive for. This should be easily implementable by just adding a column- or grid-layout with title-element at the top, per tab.
Whatever would be simple and stable. So yaeh, JSON seems to be the obvious choice. I guess Firebase or your web-framework is already using it internally?
Export is the natural downfall of every cloud-solution. Because if people have no way to get their data, many will not bother to bound with a closed up tool.