RF switch for multiple antennas
You can check out the Skyworks SKY13414-485LF chip, if you want to design something yourself. It's an SP4T antenna switch that could work in your application. Insertion loss is 0.5dB maximum, so if you absolutely want +30dBm output power, you need a reader with an output power of at least +30.5dBm. In reality you probably need more like +33dBm, because there are additional losses for filters and transmission lines. Maximum power the chip can handle is +37dBm.