How can I make my PC use DisplayPort over HDMI during boot?

Make sure to set your DisplayPort monitor as the main display in Windows. That way, when you boot, that’s the screen you'll log into, and the other screens should stay black.

Have you tested plugging the DP monitor into different DP ports? It can sometimes help with detection issues.

Check your BIOS settings; there might be an option to select the primary output. This generally applies if you’re using both an integrated GPU and a dedicated GPU, though. I’ll look into it, but from what I’ve seen, there isn't a straightforward setting just for picking an output at boot.

You might want to try different HDMI ports on your graphics card if you have multiple options. Typically, GPUs initialize the output that activates first, which seems to be your HDMI in this case. Some GPUs might have a primary port setting which could help, but it's usually in the manual. A foolproof solution is to just unplug the TV before booting up. You could also check if there’s a setting on your TV that keeps it off unless it receives a signal, but that could be a hassle.