I have a Mi 9 and it has been draining battery using the official MIUI 12. I also got a lot of ads in the system apps, which I never can turn off all of them no matter what adblockers I installed. I got frustrated and desperate, so I tried out the famed custom MIUI 12 from Xiaomi.eu. They are very good in extending my battery and so smooth and sleek. I recommend this for other phones too, since this is a custom rom. You wouldn't regret!

Warranty is still intact because I didn't open anything. Just unlocking the bootloader wouldn't void warranty as well, but u may need to wait for some time for Xiaomi phones to be unlocked due to their policy. It is very intuitive with its gesture and it has system wide dark mode, how great! It has all the modern features in flagship phones, except changing screen refresh rate as far as I have tried out.

Due to my probation status, I can't reply as often. Hope it helps.