[Realme XT ROMs, Kernels, Recoveries, & Other Devel] duLouser:
It has been a couple of months since I went through the whole process, so it's not going to be a perfect account, but it goes something like this:

Backup all relevant data

The whole process will wipe the non-volatile memory of the device multiple times, so make sure anything that you need to keep is copied to your computer before doing anything else.

Install the ADB tool-chain to your computer

You can find detailed instruction for this here.

It theory it works on all three major OS variants, but iirc. on Windows you need to install additional drivers and whatnot. It works probably the best on a Linux based systems, but you can get it to work on Windows too. I just haven't done that in half a decade, so I cannot give you any detailed feedback on that. Similarly, MacOS is also outside my expertise I'm afraid.

When you installed it properly, you should have access to the "adb" and "fastboot" commands in the command line. Note: If you can later not find the device via adb then you're probably missing the driver,

Unlock your bootloader

Unfortunately, as this is a process you usually do only once per device, my memory regarding this is especially clouded.

Activate developer mode if you haven't done this already

To do this, tap the build-nr entry in the settings 7 times.

If you have trouble finding the entry, this might help.

Turn on "usb debugging" in the developer settings submenu that you just unlocked.

Turn on "oem unlock" in the developer options, the device should reboot now

Install and use the deeptesting apk

Download it here (onto your phone)

Install it on your phone (if this doesn't work you need a different apk :/)

Open the app and tap "Start Applying"

Wait up to 5 Minutes for your application to go through

If it succeeds, good!

If it doesn't, make sure you have a working internet connection. Otherwise, you might need a different apk.

Connect your device to your computer via USB (use a cable that is capable of data transfer, obviously)

You can if your device is available by executing the "adb devices" command. On windows it's "adb.exe devices", or if you haven't added adb to your systems PATH environment variable make sure you execute the command line or power shell from inside the directory containing the "adb.exe" (Shift+Right-Mouse->Open PowerShell here should do the trick)

Spoiler content hidden.

Tap "Start In-Depth Test" in the deeptesting app.

It should reboot into the bootloader. (If it didn't, do it manually or adb commands [./adb|adb reboot bootloader])

Type "./fastboot oem unlock"(Powershell) or "fastboot oem unlock"(CMD)

Head back to your phone and select "Unlock the bootloader" (Volume button as selection navigation and Power as confirming choice.)

Your phone will reboot into recovery for formatting and it will reboot back to the system upon finish formatting.

Your phone now has an unlocked bootloader. Note, I've used the following guide as reference: guide

Notes:I remember faintly that I had to try different deeptesting apks. I should still have it lying around, but unfortunately it's on a pc I won't have access to for a couple of weeks. Still, that could be because I have a "RMX1921EU" and not a regular "RMX1921", so maybe the deeptesting apk linked in the guide is suitable for your device.

Install the TWRP Recovery

Download it here

Make sure Developer Mode is enabled (tap build-nr 7 times if it isn't)

Make sure USB-Debugging is still enabled in the Developer settings

Your device should needs to be connected to your computer via USB for the following steps again

Open a command line with access to "adb" and "fastboot" commands (on windows, this once again means opening a cmd/power shell window with Shift+Right-Click->Open PowerShell here inside the folder where you placed the adb tools.

Execute "adb reboot bootloader"

You should be in fastboot mode now

Copy the twrp-image into the same folder as your platform-tools.

Rename the image to "twrp.img"

Execute "fastboot flash recovery twrp.img"

Once that is finished, execute "fastboot reboot"

Once it shuts down, hold Power+Volumne Down so the phone doesn't boot the stock rom (not sure if that's necessary in our case, but better safe than sorry)

use vol up/down to select "recovery", then press power to boot into the recovery

the first time twrp starts it should make sure that the stock rom won't delete it

reboot into system, and then turn off the device and turn it on again with power+volume down and boot into recovery to verify that twrp has indeed not been overwritten

you should now have a working twrp recovery on your phone

Preparations for installing the rom

Download the latest rom here

If you want to use Google Apps (Gapps) you should also download them here. If you don't know what Gapps are then you should defenitely download them ;) Which apps are included in the different packages is listed here.

If you want to flash other apps like Magisk (flash after first boot), F-Droid Privilege Extension or other images then you should download them too.

Download the Realme UI 2 image, henceforth only called RUI2.0

Now you have to copy all the images to your device. I'd recommend placing them on a SD-Card if you have one as you should not wipe SD Cards during install, so it can be quite handy. Also, if you encrypt your main partition twrp cannot read images from them (at least in my case), so using an sd card is preferable, but not necessary.

Installing crDroid

Boot into TWRP (reboot, power+vol down, select recovery you know the drill by now...)

Make sure to have write access enabled ("Swipe to Allow Modifications" slider at the start)

I personally execute a complete wipe before the rest of the install but that should not be necessary, and if your zips are in your data partition that obviously is counterproductive ;)

Wiping cache and dalvik cache should be done though, if the following install process fails a proper wipe could help (caches & data, if you got no sd card you can use "adp push" to send files to your phone after the wipe)

Click install

Maneuver to the folder where you placed the zip files

Select the RUI2.0 zip

"Add more zips"

Select the rom zip

The same way add more zips for Gapps, Magisk, fdroid etc. if you want them installed too

Install

Reboot

If no error arises, you should have crDroid 9 installed now, if install fails you might need to wipe cache and data before installing

As said before, this whole thing is mostly based on my incomplete memories and there will surely be a few mistakes in it, but the overall structure should be roughly correct. I welcome any form of criticism.