tones
Hi Gordy.
Having been involved with CTCSS tone implantation on existing ground radio systems over the last 30+ years, the last radios RX to be tone protected is always the base\ ground control, hopefully at least the controller can hear all transmissions.
The 2 stage changing of radios is a real nightmare + costly, & leading to further chances of faults developing.
The other problem is CTCSS or DCS does not remove the interference only masks it, if you share frequencies you will get break through unless the tone delay times are VERY VERY fast, & interference level low.
The nuisance level to user will be reduced, but you can still get heterodyne, and the break through of the interference voice, data, or just ambient noise.
Digital is the way to go but that has its own problems with coverage, cost, & implementation