[Renate]

You could try using the Unicode U+200E (LTR mark, ‎ ) and U+200F (RTL mark, ‏ ).
Since in Android Ctrl, Shift are modifiers, I don't think they can be easily used just by themselves without modifying some code.
You can make Ctrl-Shift-LeftArrow be a directional mark. (DPAD_LEFT)

key DPAD_LEFT {
   ctrl+shift:      '\u200f'
}

key DPAD_RIGHT {
   ctrl+shift:      '\u200e'
}

I just tried that. The keycodes come through correctly.
When I hit the keys it does wacky LTR/RTL things.
(Full disclosure: I don't really understand how LTR/RTL layout works.)

You'll need Magisk or root or a loadable keyboard map APK.
What language are you typing in?