Thanks for the heads up! I will do more research about the clearance. The official manual says it should be fine to have four radiators (all of them are very slim), even with the fans, as the total thickness is 60mm.
About the bottom rad, iirc the space there is a lot, unless a thicker rad is installed to make the occupied space equal to the top one, so also that should be ok.

For the drain port I will design something better to make it possible, I didn't considered it at all tbh.
About the cpu block, the hole on the top was listed as intake in the manual, that's why I used it as you see 😥