I think the minimum RTT delay in 5G can be a little over 0.5 ms owing to minimum processing delay of 0.2 ms and 0.3 ms at gNB and UE respectively. Secondly, the gNB doesn't send ACK/NACK for UL data in 5G. If data is successfully received, the DCI field indicates new data in the subsequent PDCCH, otherwise it will indicate retransmission. Anyhow, this kind of implicit acknowledgement can be in the subsequent slot of same subframe. For example, for 30 KHz SCS, there are two 0.5ms slots in a 1ms subframe. Now, if the UE sent data in the first slot, the gNB can indicate retransmission or new data in the next 0.5ms slot of the same subframe (provided gNB complies with 0.2ms processing delay requirement).