Filling a DIY AIO loop

Hey! :)

I recently built myself a new system in a DAN A4 H2O and used a Modultra LoBo and a XSPC TX 240 for a CPU-only loop. The radiator is directly connected to the CPU/PUMP-Block, I use Koolance QD3 fittings so I can disassemble everything without having to drain the fluid.

There's not much space in this case so I can't use a regular reservoir which makes it quite challenging to fill the loop. At first I thought that I could simply pre-fill both sides of the loop and then just plug them together. But that seems to leave quite a bit of air which now wildly recirculates even at low pump speeds.

Now I need a better way of filling this loop and have not found a good solution so far. That's why I wanted to ask for some tips here, maybe you guys can think of a better way to handle this?

Would greatly appreciate any input. :)