How can I safely disable Intel RST?

You have to turn it off in the BIOS, I have seen the option in MSI laptops but not in Dell laptops, so I would assume it depends on the manufacturer and probably the model too.

For example, these are the steps for Acer Aspire 3 series:

If you find software incompatibilities with Intel Rapid Start Technology (iRST), you can disable this energy-saving sleep mode in the BIOS:

Power on the system. As soon as the first logo screen appears, immediately press the F2 key, or the DEL key if you have a desktop, to

enter the BIOS. Use the ARROW KEYS to navigate to iRST. Use the ARROW KEYS to select iRST Support. Press ENTER to change it to Disabled. Press the F10 key to save changes and restart the system.

NOTE

These BIOS options are only available on models with a conventional hard disk and additional 20GB SSD. It is not possible to configure iRST in the BIOS for models with a single large SSD.

Source: https://za.answers.acer.com/app/answers/detail/a_id/16631/~/disable-intel-rapid-start-technology