Toilet Still Gurgling
If there is not any backup evident at the outdoor clean-outs or tub drain when sinks and toilets are flushed together, then I would say the septic is not slow draining and air bubbling into the toilet requires that the soil vent serving the toilet be plugged some other way.   Since the toilet vent line was recently cleaned and found to be plugged, I would surmise that the vent has somehow become plugged again.

Offhand I can think of two possibilities:

(1)  The toilet soil pipe has a sag near the connection to the soil vent such that the branch to the vent tends to collect sewage and become plugged;

(2)  The toilet soil vent is connected to the drain line incorrectly (incorrect wye, installed backward, etc);

There are companies for hire that can run a camera through the lines to find the problem if you are so inclined.