How close to P1dB can I go?
It all depends on how much distortion you can tolerate, and any other side effects that may occur when saturating the amplifier (harmonics, etc.). You might be fine running several dB over the P1dB point. Or maybe you need to stay several dB below. You might just need to experiment. If your FMCW radar can tolerate some distortion, then you might be able to hit it pretty hard. On the other hand, if you're using a digital modulation like high order QAM, you might need to stay well below the P1dB point.