After the Pie update, I succeeded by installing the new "HAL3_enable_MIUI_Pie.zip" under TWRP, my phone is not rooted ... HAL3 Enabler Link (thanks to miui.blog)
I tried various Gcam mods, all work...
Plenty of Gcam ports work in Pie folks you can find good APK's here. Download "60fps_miuiPIE_stable_v3_stereo.zip" and install via Magisk to enable SlowMo and "GCam_6.1.021_Mi6_Edition_5.5 +.apk" to install the APK. They make APK's especially for the Mi 6/Mi Mix 2 and I've found them to be the best Gcam mods for this phone. They update them quite often too.
Use the HAL3 enabler that @Simsphil posted and you're sorted