I have similar configuration as your in same case Lian Li PC-O11 WXC.
You need to install drain port on lowest part in loop.
Best on radiator with Male to male rotary and X spliter to separate loop from drain part.
You will need two male to male rotary because then you could turn left right ball valve.
This is maybe best example...
Draining loop is not complicate if you have right tools.
Transfusion system drain loop very easy. You need one transfusion system and one 20-50ml Syringe with standard end.
Because that tube is capable to fit on syringe without needle with just little effor. And it's enough thin to go deep in system.
I will use this kind now to drain loop and add ball valve.
Off course you need to cut all of these plastic parts and to use common syringe not from picture.
And little by little you can draing loop completely. Actually if you have 50ml syringe then faster. I use 20ml I didn't found bigger.
I tried and I pushed him completely up to monoblock.
And I didn't resolved for me. I will use 2 x 90 angled adapters and to move inlet port from top of bottom and somehow to install ball valve.
I'm limited with space as well. But I will menage somehow.
But I swear in god I will not buy any more ASUS products if I don't find ASUS ROG-TUF VGA Holder. I use Styrofoam until find him.
My drain mechanism for now.
Than when I resolve ball valve everything will be easier and I will probably switch to hard tubes because today people pretend like you don't have loop if no hard tubes. That only push me to use glass and to laugh them.