[Magisk] Mirardt:
After lots of people here and on Reddit asked me to make a guide, here it is.

DISCLAIMER: DO AT YOUR OWN RISK. YOUR DEVICE MAY BRICK. I AM NOT A DEVELOPER, JUST A TECH NOOB WHO WANTED TO MAKE GOOGLE PAY WORK ON HIS PHONE.

Although the main point of this guide is making Google Pay work, a byproduct of going through it seems to be a better performing and more stable system (at least this is my subjective experience).

If anybody notices any mistakes or misunderstandings on my part, please post it so i can correct/update!

Step 0: Delete ANY kind of Password or Protection from your phone. I had multiple instances of TWRP not being able to decrypt my phone data because of a wrong password. I have no idea why and assume that this won't happen with no password set up.

Step 1: Set Up an Xiaomi account on your phone and start the process of unlocking it in Developer Settings. During this process, you have to download the Unlock Tool and probably have to wait around 10 days.

Step 2: Download this and unzip it. Also Download GApps Pico from here.

Step 3: Install TWRP via ADB as it is descripted here. Use the "recovery.img" you downloaded in Step 2.

Step 4: Boot into TWRP and change the language to English. Then make a backup and safe it on your PC by pressing "Mount" and then "Mount USB Storage". Move the files "Magisk-v20.4.zip", "K30U-去vbmeta校验.zip" and "open_gapps-arm64-10.0-pico-20201014.zip" you downloaded in Step 2 to your phones storage.

Step 5: Wipe your device by clicking on "Wipe" and swipe to "Reset". Then flash both "Magisk-v20.4.zip" and "K30U-去vbmeta校验.zip" by clicking on "Install", selecting those files and swiping to flash them. You MUST flash both of them or your device may brick.

// At this point you might be able to do what I describe in Step 6 by using Advanced/FileManager without ever leaving TWRP. This would speed up the process but its a bit more messy and for me sometimes it works to deletes System Data and sometimes it doesn't. I have no clue why and if you're up to messing around with it you probably know that you can and if you don't know its probably best to go straight to Step 6//

Step 6: Boot into system and rush throug config since we are going to reset the system again in Step 7. Download and install the newest version of MiXplorer or any File Explorer with Root Access and navigate to root/system/data-app. Unfortunately this is the part where i can't exactly tell you what to delete since its already gone on my device. I deleted around 5 apps and I remember they were among the first 10 apps listen. The apps I still have there are the following: Calculater, CleanMaster, com.zhihu.xyz, Email, GameCenter, Huanji, MiDrive, etc. so you can find out which ones to delete (please post the names of those you deleted here if you can so i can update this very messy Step 6). Importantly, you need about 200MB free space, you can see this in MiXplorer.

Step 7: Reboot into TWRP and repeat Step 5. After that, flash "open_gapps-arm64-10.0-pico-20201014.zip". Boot into System.

Step 8: You should be greeted by a friendly Google Setup instead of the Xiaomi one. Yay! Set the phone up and download GBoard, a Terminal app and GPay from the Play Store. Launch Magisk and Update the Manager, but do NOT update to Magisk 21.0, the phone can't handle it. Enable Magisk Hide and add all Google stuff to hide. Also hide the Magisk Manager. You should already pass the google Safety Net, but there is something left to do to use GooglePay.

Step 9: Download the Magisk Module MagiskHide Props and both change your fingerprint and force basic key attestation to "Redmi K30 Pro" inside your Terminal app by typing "props". Other fingerprints probably work, too. If you need more help, look here. Reboot.

Step 10: Open GPay and set it up. It should say "Hold to read" Go to settings/Connection&Sharing and change you default wallet to "Use HCE Wallet". Go to Tap&Pay and select Google Pay as default wallet.

Voila! It should work now.

Bonus step for a nice system: Deinstall as much Bloat as you can/want normally and with this tool. The rest, which you can't deinstall, deactivate with the tool.

Credits and Mentions:

[MENTION=7786725]zgfg[/MENTION] for the huge support figuring MagiskHide out and helping me with stuff that is way too complicated for me and way too easy for you. ;)

[MENTION=4667597]Didgeridoohan[/MENTION] for making MagiskHide Props and helping out!

[MENTION=1690962]JaboJG[/MENTION] for helping me out a alot early and making me aware of the custom TWRP and Magisk for the K30U

[MENTION=5419391]Prprdog[/MENTION] for making a Custom ROM guide that mentioned Gapps Pico and that it makes GPay work.

[MENTION=9526863]nikitos2323[/MENTION] for making me aware of the guide