Surely we have enough frequencies left over for each aerodrome to have its own discrete frequency rather than a big multicom?
And for places where multiple strips are in proximity just give them an area multicom.
At 5000 feet, VHF is about 60NM.
So let's say any given place has 20 discrete frequencies dedicated to high level comms and you can't reallocate them within 400NM, this leaves
At 25Khz spacing, we have 760 channels to choose from.
At 50Khz spacing, we have 360 channels to choose from.
How many strips exist?
Country airstrip guide has a total of about 1900 airstrips.
NSW about 310
NT has 182
QLD has 595
SA has 178
TAS has 64
VIC has 143
WA has 382
"AOPA Airfield directory" says about 2000 (ERSA plus unlicensed).
At 25Khz spacing I really think we have enough for every airstrip to have a discrete frequency including "farmer joe's paddock".
I'd be happy with mandatory radio calls required for operations at an aerodrome plus ops 0-2500ft AGL within X NM of aerodrome are required to tune into said aerodrome.
Really we need avionics to come into the 21st century where you just press a button for GPS-linked "choose suitable frequency button".
Or, better yet, a transmit-and-store mechanism built into avionics that lets multiple people speak simultaneously and transmissions "keyed" by various items such as callsign (eg. adsb ID). Aircraft have two transmit buttons - one for ATC direct comms and the other for broadcast. Let's also have a "replay" button in case you want to re-hear that again.
But I digress!