The data transmission rates from MRO are not that bad, around 2.0 Mb/sec when I looked earlier.
For reference, Voyager 1 is only 160.0 b/sec:
View attachment 452108
Most of the comms are over either x-band (8-12 GHz) or S-band (2-4 GHz) frequencies. DSN uses lower ends of both.
Downlink data transmission for DSN Telemetry Packet Service goes upto 10 Mbps for deep space transmission, and 150 Mbps for near Earth Ka-band.
DSN does support Multiple (2+) Spacecrafts Per Antenna (MSPA) for downlink, as long as all devices lie within the beam width of a given antenna, and transmit on different frequencies. For uplink, only one device at a time can receive the commands.
More data here:
https://deepspace.jpl.nasa.gov/files/820-100-F1.pdf
Unfortunately, the constellation is around the wrong planet to be of help to Perseverance.