For modifying the lk I needed some useless space closer to the required address range. So I utilised the unused function "aging test mode" that is present at the address 0x4c404460.
Then I modified those codes like this refer above image.
So normally it works like,

Some function ---> other function

This moded version works like,

Some function ---> my code ---> checks for volume up ---> other function

Modded and self tested (Realme Narzo 30 4G RUI3) by @antagonizzzt

Also check: recovery flashable zip

moded LK for RUI3
Volume up ---> Recovery mode
Volume down ---> Fastboot mode

How it works?

What's new 🥳

1. Added a new thread.

Menu-like feature that runs along with fastboot mode you can just trigger these
Functions while in Fastboot (i.e no need to force power off)

UP - Continue to System
Down - Boot to Recovery
Power - Turns off device

2. Removed/Patched fastboot restrictions like

FAILED (remote: 'not support on security')
fastboot: error: Command failed

So some restricted commands will work

How to flash?

fastboot flash lk lk-rui3-menu.img

or

Recovery flashable version

Screenshots

Features:
Same features from RUI3 version
Recovery flashable version
Screenshots

Minor change
Can also boot into recovery (up + power)

How to flash .img files without recovery flashable?
Go to your existing fastboot and enter the following command

fastboot flash lk lk-rui2.img

Or make recovery flashable by replacing the lk.img in this zip with this (rename lk-rui2.img to lk.img)