If it's not clogging up over quickly then isn't a case the enzymes or whatever are doing their thing?
It took me 18 years here to actually find out where my cess pit is and that's only because the loos were backing up. A pink toothbrush seemed to be the culprit getting wedged sideways and acting as a wet wipe magnet. I had it emptied anyway when I found the access hatch but left a good slug in the bottom in the advice of the chap who said you don't want to lose all the "good stuff".
The DNO made me PME without me even knowing. Always happy with a rod the house-sitter called them when a tree came down on the overhead.
Arrived back from holiday to find a new connection from the pole and a PME sticker on the head.
So your neighbours sh!t on your lawn and you're a bit handy with woodwork. Payback time and problem solved all in one. Try and aim for their picture window:
You say that but we've three new neighbours this end this year and none are knocking down / rebuilding. Presume they'll just go for a 50% extension if they can.
If I had the dosh I wouldn't bat an eyelid flattening the whole place Including the bathroom.
I lined the stud wall containing my Geberit wall hung WC frame & cistern with 2nd hand acoustic foam tiles. Near silent filling and flushing (once "it's" gone out of the pan).