Well, of course, you don't have to delete the file, but then you'd have to make sure the filename was different so that it would trigger "adds an item" (I'm assuming by the name that "an item is changed" would not trigger it). Presumably Keyboard Maestro can delete the file as part of its macro, so that shouldn't be too onerous.
Here's the problem that you're going to run into if you try a more 'straight forward' approach.
Once you have left your home Wi-Fi network, you and your computer are no longer on the same network, Yes, that seems obvious, but bear with me.
So, if you want to interact 'directly' with your computer, how are you going to do it? SSH? Some sort of a web-trigger?
How are you going to connect from outside of your home Wi-Fi network to a computer inside it? Open a port in your router/firewall? Does your Mac have a static IP address?
You can do that with your eero system. I've done it myself, and it works fairly well, IMO. But it's another thing to manage. And sometimes it doesn't work. Because reasons. Is your Shortcut going to be able to handle repeating the request a few times if necessary, but stopping once it actually gets through?
The nice thing about creating a file in a service like iCloud or Dropbox is that it's quick, easy, and reliable. Apple is very much motivated to make sure that files work well with cloud storage, especially iCloud. That's a core feature that they really want to have working well.
Once the file is added to cloud storage, your Mac wants to download it, even if you have 'optimize storage on' I believe this should work if Keyboard Maestro is just looking to see if the folder has had a new file added to it. I assume an iCloud placeholder file will work if you don't choose 'Ignore Partial Files'.
So iOS is going to work hard to make sure that the file gets uploaded, and macOS is going to work hard to make sure that the file is available. Both of those things increase the odds that your 'roundabout' method of triggering this action is much more likely to actually happen.
Otherwise, here's how I look at what you're relying on: a SSH connection through your firewall to run a shell script to run an AppleScript to execute a macro.
Having done a bit of automation in my time, I know which one I'd choose.
p.s. - I would not recommend a Bluetooth trigger. Bluetooth is… unreliable, at best. I've tried Bluetooth-based triggers before, and have seen these two things happen:
I've been sitting at my desk with my iPhone next to my MacBook and had my Mac say "Oh, your iPhone has left the area, I'm locking myself."
I leave my office, cross a large room, go into a bathroom, and my Mac still thinks my iPhone is nearby.
Logical? No. Actual? Yes.