@J65nko
thank you forever! You pointed me in the right direction with your last post: I simply re-initialized the BIOS Setup to its original default specs and it worked (I probably must have changed something in the BIOS during my first attempts to make a reboot...). Now I can reboot to OpenBSD freshly installed, X-windows manager included
Ok, I think we can consider this whole thread solved. Now it's my duty to learn OpenBSD in depth.
Once again: awesome support! Thanks!