I hope you aren't still useing that plumber if he can't solve your problem. I have 1.6 gallon per flush toilets that clear the bowels every time. I can't direct you to where there is a kit that flushes multiple times but I am certainly willing to try and help you as I'm sure others are as well.
If you dump a 5 gallon bucket into the bowel,does it clear?
When you flush,do you hear gurgling from the shower or tub drain?
When someone flushes and showers upstairs,is there disturbance of water standing in the lower toilet? What about gurgling from lower tub/shower and sinks?
Have you looked down cleanout for standing water in line when you are experiencing trouble?
You said "septic",are you not on a public sewer system?