Just a recap:
Arctic LF 2 240
Ryzen 3600X
Phanteks P400(non-A)
Now then...
1)You made sure the paste spread and covered the cpu IHS fully? The little in the middle application tends to not be too effective on Ryzen due to the multiple chiplets beneath.
2)Arctic LF 2 + P400.
Goes without saying that since the only option is to mount it up front, you're forced to mount the cooler right up against a rather hard airflow restriction.
The best way to maximize airflow here is to position the fans behind the radiator, pulling air inside the chassis. Just a reminder of which way air comes in and out:
3)About the LF 2: Boxed unit ships with pump and fans managed by single PWM splitter.
"While the Liquid Freezer II 280’s 140mm fans are factory installed and managed with the same PWM power that controls the pump and auxiliary fan, we opted to disconnect the fans and run them independently for our tests, allowing for independent PWM cooling fan management while the pump remained powered at 100%."
If you want to control the fans and pump separately, you'll have to connect the fans directly to the motherboard, otherwise, all 3 will operate on a preset PWM curve.
Fractal Design's Celsius does something similar.