I had a similar condition for AC, ended up the blower motor resistor was shot, highway speeds evidently pushed enough air to make things work. Cheap and easy fix. Make sure your blower motor is actually working.
Re-check the coolant level in the radiator and coolant reservoir, a low coolant level will result in the heater performance that you are experiencing. If the cooling system is full there may be a restriction in the heater core.
The a/c is fixed? Previously it did not work at idle/ low rpm. You said they fixed it so the compressor turns off at idle. That would not make it work better at idle.
I believe the OP is indicating that compressor shutoff at idle is the way the car was designed so the problem isn’t “fixed” but is now understood that this is the way the car is designed to operate.