Seems a bit over complicated. Why not just design a 5W 50 dB pi attenuator and make the input resistor able to handle 5W? A 50 ohm resistor to ground 7.9k series and 50 ohm to ground. You can make a 5W load using a 50 ohm SMD terminating resistor - e.g. Anaren C10A50Z4 good for 10W up to 3 GHz. Make sure there is sufficient heatsinking to the ground plane - lots of vias.
Some rules of thumb to make attenuators:
- components must match frequency ranges - GHz without SMD is hopeless, Multi-GHz needs coaxial architectures
- no components very far off 50Ohms, otherwise their parasitics dominate and flatness is gone.
an 1Ohm resistor is an inductor, a 10K resistor is a capacitor more than just a resistor at high frequencies.
- no stages with more than 20, better 10dB. Cascade stages if you need more attenuation
- watch power handling in the input stages
Special good reason why you need to build ? No ? Buy one from a renowned source, problem gone.