RX_ENC and TX_ENC are used to program the encoder, it is not usable for rotational information. The four A and B channels provide the encoder’s specified number of output pulses per revolution, typically 1024. The Z channel provides one pulse per rev referred to as the TDC pulse, when the shaft is at a specific angle (per the shaft flat).
The LTC489 or something similar with four receivers will do the job, add a second one if you want the Z pulse. It all depends on what you intend to do with the signals, something which you haven’t explained. More importantly, why do you think you need an encoder when using a stepping motor?