Redirection after adding new groups and gnodes using group module
If The issue you posted about promises to solve your problem then your best bet would be to test the patch (always on a test server) and help this issue forward so others can take advantage of that in the future, because this is how drupal progresses mostly.

Just download the latest .patch that you find in the group modules directory 
and open up up a command window (in windows) or shell in linux based systems and apply the patch 

in a linux based system you could run the following commands, assuming group is in modules/contrib/group and using latest patch as an example 

cd modules/contrib/group
wget https://www.drupal.org/files/issues/2817109-by-rachel_norfolk-ericras-How-to-redir.patch
git apply -v 2817109-by-rachel_norfolk-ericras-How-to-redir.patch


If the patch does not report any errors you will probably get a configuration field to do what you are asking and it would be great to post in the thread how the patch went for you.

More on patches here

If you want something different you should create your own module and implement a hook_form_alter to set setRedirect for both of the forms that you mention similar to what rachel_norfolk does in comment #3