You are basically creating a repeater.
There are any number of repeater controllers that might work. NHRC comes to mind.
However they are on COVID lockdown until whenever.
Basically you need to extract a carrier operated switch (COS/COR) from the AM Aviation receiver. That signal will come from the squelch circuit. Then you need to tap the audio from the receiver. The controller will generate a PTT signal and mike audio to feed into the CM300 D.
Setting levels will be the important part to make sure the signal is not distorted.
You will probably have to put the controller and receiver into a shielded box to keep the UHF radio from tearing up the performance of them.
If the air channel you are repeating has a lot of traffic, the CM300D may not tolerate the duty cycle. You should set the time out timer to protect the radio.
Since you have mentioned AVTEC and SMART PTT , I assume someone has proposed something to you which may not meet your budget, but might be more suitable to your needs than the solution I have proposed. I am assuming your requirement isn't critical life safety or has a critical business function.