You can temporary press F7 to switch to TEXT mode.
This most probably something about the screen resolution. By default Ventoy use 1024x768 resolution which maybe not fit your monitor.
You can try to set another default resolution in theme plugin.
Yes, unless you like managing secure boot settings. Ventoy does support secure boot when enabled during setup, but that requires you to enroll a custom crypto key as described here. I didn't test it, and it sounds like the author himself doesn't like it either. The easier option is to just disable secure boot when booting from USB (not just for Ventoy, many live OS don't support secure boot).