One caveat I’d throw out there:
But that COULD return a lot of activities — as most - perhaps all? — :filtered conditions are done in the browser.
So a couple of thoughts: If the Activities to Search are only those created by current user (or some other specific user) put the Created By constraint in the Search. (This is redundant if a privacy rule forbids access to other Users’ Activities, of course, but I thought I’d throw it out there!)
The other thing is you COULD do if you want to keep the search all server side is to build an API Workflow (where you can have server-side :filtered … Advanced: constraints) and then call that endpoint via the API connector, so that you can actually get a return value.
(And why oh why does Return Value from API only work when calling via the API Connector? There’s so many applications [like this one] for return values inside of Bubble. So goofy!)