How can I get around Intel RST thingy while trying to install Ubuntu?

This problem will probably be solved sooner or later. I will try to explain what causes the trouble.

Linux kernel properly supports RST mode. If you install Ubuntu to an empty computer with RST enabled in UEFI, the ahci kernel module shows [RST mode] and nothing bad happens.

Also you can configure a raid in Ubuntu and register it with RST too ;-)

But if there is Windows that makes some benefit of the RST raid features, it makes some trouble for grub. Windows can be configured a few different ways to use RST raid. At the moment there is no automatic way to detect it.

Sooner or later extra features should be added to grub and the problem will be fixed.

Now many vendors removed AHCI mode from UEFI settings, because many users switched it, couldn't boot Windows and complained. Some manuals you could find online say how to fix Windows registry to boot in AHCI mode, but this is not an option on many systems.

I am not a Windows expert, but I can suggest to re-install Windows in AHCI mode. Most likely there will be an option when you install the system. After Windows is installed without any raid, then there should be no problem to install Ubuntu.

In addition a few words about RST itself. It is not a hardware raid. It is a UEFI extra feature that allows OS to tell UEFI what sort of raid it is using. So it doesn't give any performance boost. It is useful only for Windows boot loader.

So the problem is not RST itself, but Windows installed on some raid.