I've used communicante to host our annual summer retreats in years past - they have been professional, responsive, flexible with scheduling and easy to work with from an admin perspective.
The product itself has gotten better over time but there are still features that could be improved (ease of use). We're using it now again this year at my current job - we love how user friendly everything about it is! It makes hosting events like these so much easier than putting them together ourselves or paying someone else to do it all by themselves.