Thanks for your interest in Syncfusion products.
Based on your update, we can see that you have tried this below option to add additional rows inside group to display at each group end.
But our client report designer doesn’t displays the output as expected though the option tried is correct. We already have plans to improve this Tablix usage in our March 2019 Reports platform release for Client Report Designer which will resolve this reported issues.
Additionally, for upcoming December 2018 Reports platform release we have implemented new table control for our web Report designer which will achieve your requirement without any issues.