I'm also with RR and never had less SOT than MIUI. Right now I'm with 12 applications in background and no problems. It's also much smoother than MIUI. Tried miui 9 to test but it's almost the same as miui 8. For now I'll stay with RR.
Assuming that you have unlocked bootloader, enter twrp and wipe data / cache / dalvik. I'm not sure if you need to wipe system also (confirm that). I'm with Resurrection remix and it's much better, but I'll give a try to Miui 9 soon... :)
The process works, you just have to wait 3 or 4 days and with officially procedure you just have to do it once, not the same about unofficial method. I don't understand why do you blame xiaomi.eu, they don't lock bootloaders.
Install 6.12.22 with twrp. Then install 7.2.23 via updater app. After that your bootloader will relock again. Then you have two choices : unlock unofficial way ( and do it every time you upgrade) or official way ( unlock with miunlock) and don't have to do it all the time. Your choice...
You must download the last xiaomi.eu rom (7.2.23) and put it on your phone. Then go to the updater app and press the 3 buttons (upper right) and choose "select update zip file" (or something similar) and update. Then unlock your bootloader "officially way" for future updates that can be done...
When you upgrade from 5.1 to 7 it will relock the bootloader. If you proceed the unlock "officially way" it remains unlocked every time you do the upgrade with xiaomi.eu roms.