re: A/C periodically having very weak air flow

Sounds to me you are on the right track to isolating the potential problem possibly being with the air handler/furnace blower motor. You hearing any high pitched sound from the motor possibly indicating a bearing issue? One can check the amperage draw on the blower motor and compare to it’s rated load amps to isolate it as potentially on its way to failure.

Certainly low air flow across the coil caused by a failing blower motor can cause it freeze up and reduce air flow, then as you say, running HVAC in fan mode only will hasten defrosting of the coil.

Alternatively, you could be slightly low on refrigerant and the evaporator coil is freezing in late in the evening which again will restrict air flow through the coil while it is frozen.

Anyway, a HVAC tech will need to diagnosis, but you really don’t to keep operating the unit in this fashion too long and potentially cause more damage to the unit.