Does anyone know if it's possible to do a multiselect/checkbox HierarchicalMenu using Instantsearch? Or a custom hacky solution with nested RefinementLists maybe?

You’d have to build a custom widget like this: https://discourse.algolia.com/t/hierarchical-menu-widget-using-checkboxes/2955

yeah i've seen that thread, they cannot solve it there either. and algolia's response is just "As of now we do not support multiple selections in a hierarchical menu. The implementations is tedious and will require a significant number of request operations. This is not in our roadmap for the next quarter. I’m sorry"

Hmm, I vaguely remember at least one user showing an implementation like this with instantsearch and a custom widget using connectors