Hi @jippeheijnen,
Since Bruno is currently unavailable, I will share some information I gathered from our R&D team. The first solution they pointed out is to get a wic image from your build. I see that you have already tried that out and facing some issues. I think Bruno will be able to give help you unblock on that topic when he is back.
The other thing is, a workaround: using a signed bootloader to load Toradex Easy Installer and then just flash signed images as normal. This is a thread that explains the procedure: Toradex Easy installer downloading failed on closed device (HAB)- Colibri-i.MX7
Do you think that could be a solution for you?