There is only 1 120 mm rad with a cpu and two GPUs? Don’t do it. Buy a new case, wider tubing, new fittings, new pump! I honestly think you should get a new case and just start from scratch. There is two many different diameters of tubing and different fittings.
so to fix that- get a new case, new liquid cooling parts. Also take a part the loop and remake it outside the case to leak test that gpu block.
also have at least 360 mm of total radiator space- but I suggest way more for good temps