I am working on a better answer.

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.

After confirming that a slow draining septic is not the issue by opening the outdoor clean-out and witnessing no backup and having dealt with any and all vent problems and having made certain all lines are clear, there are only two other possibilities I can think of:

(1) The drain line between the house and the clean-outs is incorrectly sloped or has a low point which fills with liquid and causes slow draining; or

(2) The drain line from the house to the septic has an obstruction or restriction somewhere due to incorrect piping.

If it were my system, an easy test I would perform would be to open the outdoor clean-outs, run several sinks and flush all the toilets together. Do the clean-outs still not flood under that condition? Does the first floor tub drain back up under that condition? If no on the clean-outs, then the septic is not slow draining, and in that case I would start looking for piping problems between the clean-outs and the rest of the house, especially if the tub drain backs up.

If water can be seen backing up in the clean-outs, then I would pull the inspection ports on the septic inlet and outlet and look for clues. For example, a slow draining septic system will often form a soap cake around the inlet baffle which is an indication that the septic system does periodically back up. Also, soap formations exacerbate slow draining and not all septic service companies look for this and clean it out. If there is a soap cake there then I would inspect the inlet and the outlet. Are the baffles/tees in good condition? Is there an outlet filter that has become plugged?

If the septic is definitely good and the house is built on slab, there are services are available for hire that will run a camera through your lines.

That is all I can think of. I would welcome someone else's ideas.

I am working on a better answer.