3 more technicians have been out to my house since my last post, and one of them finally switched my ONT to router connection to ethernet. That had been temporarily tried before, but decided to try again with permanent wiring.
It seemed to help a bit, since it was almost a week before slowdowns started again. Now it seems as if the slowdowns come mostly at night, around 10 PM. I wouldn't think that internet traffic in my neighborhood would be the busiest at that particular time of day though.
Perhaps something else changed somewhere down the line, as it is better, but still not as good as it was before I "upgraded" my internet speed.