Hyperpixel Square with second monitor + xserver, choose on which display to start app

Hi,

I have a Raspberry Pi 5 + Hyperpixel Square + a HDMI monitor connected.
I have a graphical app that needs to be started, and I don’t have the need for an entire desktop UI, so I basically boot into the CLI, and then start X server to just run my App.

So my .xinitrc is basically a one-liner:

exec <app name>

I simply run startx and this works fine: the app starts on my hyperpixel square attached to the PI.

But… there are also cases where I want to run this App on a connected display. Up until now I did this by editing /boot/firmware/config.txt and rebooting the Pi, but surely there must be an easier way than this right?

I tried tinkering with the .xinitrc file, or try adding arguments and stuff, but the best I get is that my HDMI display clones what is shown on the Hyperpixel Square - in a 720x720 square on my 1920x1080 display.

So what are my options here? I’m fine with something like an argument or editing the .xinitrc-file, just as long as it doesn’t require rebooting the Pi everytime. It’s anoying.

Any thoughts on this?