My installers are currently not code-signed/notarized.

Get an Apple Developer account for $99/year. The process of code-signing and notarising is simple.

Installers work on your own machine because it's your machine. As soon as you use the installers on another machine, the gatekeeper kicks in.

It matters how you transfer files to your test machines. If you move stuff by USB stick, it doesn't get the 'scary downloaded from the internet' flag, and gatekeeper is more relaxed.

To test installers on another machine, they have to be downloaded from the internet, to simulate what a real end user would do.