Proxmox installation stuck on "Welcome to GRUB!"

[esi_y]

After forcing a reinstallation of GRUB with a Debian 12 LiveCD it booted again,

That's disturbing in the sense that you have to expect same issue upon next kernel update though.

I will continue with the installation with the following steps:
Install the Proxmox VE packages
Install the Proxmox VE packages
apt install proxmox-ve postfix open-iscsi chrony

I'll keep you informed.
Thanks!

Thanks too! I am really interested what could be causing this. And very obviously, it actually is the problem that your original ISO install gave you too.