In environments with high client density, deploying APs for 5 GHz coverage is recommended due to the larger number of non-overlapping channels available, which can support higher data rates and client capacity.
Disabling the 2.4 GHz radios on some APs can reduce interference and improve overall network performance.
https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/86/b_Cisco_Wireless_LAN_Controller_Con

To reduce co-channel interference, it's beneficial to decrease channel bonding, which will increase the number of non-overlapping channels available. Reducing the power of the APs can also help minimize interference by reducing the range of each AP's signal, thereby reducing the chance of overlap.

An RSSI value of -77 dBm is within the range that a 5 GHz radio can receive and decode, provided it's not below the RX-SOP threshold set for the RF profile. Since the RF profile is set to Medium for 802.11a, which operates in the 5 GHz band, the AP's 5 GHz radio will decode the frames.

For a point-to-multipoint mesh network site survey, configuring the Rx power and Tx power is essential. Rx power will determine the sensitivity of the AP to incoming signals, which is crucial for understanding the reach of each mesh point. Tx power settings will influence the strength of the signals transmitted from the AP, affecting the network's overall coverage. References: Designing Cisco Enterprise Wireless Networks (ENWLSD 300-425)

The Cisco Aironet 3800 Series APs support mGig, which allows for higher throughput over existing cabling.
However, the maximum cable length that supports 5G operation is limited by the quality of the cable and signal attenuation. Category 5e cabling can support 5G operation up to a length of 130 meters, which is within the acceptable range for maintaining the required signal integrity for 5G speeds.