I added the 20dB attenuator for safety, because at initial powerup there could be a TX spike from the board initializing. I thought that you may possibly get away with 10dB because the maximum TX power is about +18 dBm depending on frequency and the maximum RX power level before physical damage is about +12 dBm. But that felt too close so 20-30dB does sound reasonable, me personally I would not connect any output to an input without forcing the signal level to always be below the level required for damage, that way you are reducing the chance of binning or damaging hardware to zero.
