Well, no : no other way.
Have to do it one by one. 🙁

Sorry 😳

But maybe it will be easier to simply "close" them.
This can be done through multiple update functionality.

And if you only created projects, and no other item referencing these project, you can try deleting directly from database : just think to also delete planningelement with refType='Project' and refId=id of project.

Or simply try to write your own php code that will do it...
Something like (sorry not tested) :

include_once("../tool/projeqtor.php"); // change this depenging on location of your script, here is may be in /test folder
$startId=10; // change with your onw start id to delete
$endId = 100; // change with your onw end id to delete 
for ($id=$startId; $iddelete();
}

Simple, no ?

Easy as pie! Sample code above worked like a charm.