Is there a free alternative to Smart Packer that would allow me to pack the _data folder into a single .exe?
Since I’m just starting as a hobbyist I can’t really afford to put down the initial money that Smart Packer costs, but would still like to have the game be a single file.
This is generally a really bad idea and very prone to errors. What it does is to pack the game into a Virtual Machine, which Unity is not aware of. It can hamper performance and usability.
I would suggest building an installer instead, with something like Inno Setup:
Which is free as in free speech and free beer.
This way you can send a single file to your players which installs the game and also uninstalls it.
If this is about protecting your assets, forget about it. It is a waste of time. Someone will find a way to unpack it, if they are determined enough.