Curiously, shortly after the presentation of his direct successor, Xiaomi Mi MIX 2 has undergone an accelerated vertiginous as regards the modding field. In fact, after receiving the official support of TWRP, the most famous and widespread custom recovery of Android, now it's up to the custom ROM par excellence.
The official support of LineageOS 15.1 has also arrived, namely the version based on Android 8.1 Oreo. Since these are built officially recognized by the independent development team, the degree of stability is decidedly high, even if several users report some problems deriving from the upgrade to LineageOS from other custom ROMs. The developer who takes care of the releases for Mi MIX 2 (mikejoannina), has therefore recommended a double wipe during the flash phase, in order not to run into these problems.
You can learn out more at the dedicated thread on the XDA forums, while this page you can download the build of LineageOS 15.1 that gradually will be published. Also, do not forget to download and flash a GApps package as they are not included in the ROM.