thats pretty much it, but if you flash official china dev, you will loose twrp again, so you have to reflash it afterwards. if you want to avoid this step, install xiaomi.eu (i said miui.eu in my previous post, that was wrong)
so to summarize:
1. Unlock through Mi Unlock
2. Download TWRP (latest version!) and flash it.
3. Download and flash latest xiaomi.eu rom (it's a debloated china dev version click
4. Download and flash Lineage OS, gapps (opengapps.org --> ARM64 --> 7.1 --> and choose a version, I always take pico because its the smallest) and magisk