Please add your suggestions to improve the documentation for the Xiaomi - Mi MIX 2S - polaris here …
It may be worth mentioning the option of separate firmware/vendor update as preparation to moving to a new major release. There is a well-known open-sourced PROJECT that extracts these components from the official Xiaomi’s ROMs to make such an update possible.
Good suggestion @e.follower
Will have it added to the documentation for Xiaomi devices.
Actually, this applies to any Xiaomi device that may require flashing of the latest official MIUI before upgrading eOS.