You sound frustrated, and I can imagine that noise or an aircraft passing overheat at a low altitude - even if that’s a legal altitude - can cause concern in the non-flying public.

If I can make some respectful suggestions:

(1) I’m not a Colorado local but did some training years ago at KAPA. The area where the airplane was doing maneuvers is a known local practice area, and would probably not meet the FAA’s definition of a congested area. The town center of Elizabeth, CO is barely even marked as an urban area on a VFR sectional chart. The most likely explanation of this is a student doing completely legal maneuvering practice (ground reference maneuvers and/or engine out simulations, both of which are required by law and are conducted between 600-1000 feet above the ground). It’s possible the student or instructor were slow in recovering, leading to a descent below expected altitude. However: it unlikely anything illegal happened here, and the FSDO is not going to do anything for you. One other note - ADSB altitude reporting can be inaccurate by several hundred feet when ambient air pressure is abnormally high or low (CO had high pressure today, so the traffic may have been 250 feet higher than ADSB reported), or if the airplane’s transponder is reporting inaccurately (+/- 200 additional feet is possible).

(2) the Quiet Skies people are pushing training flights further and further from the airport - which means small towns like yours are more likely to get get training traffic, not less.

(3) what I would do in your shoes is call the flight school and ask to talk to the chief pilot. Explain your concerns and share the time/tail number so they can look up the ADSB track. It may be that the CFI needs retraining or that the chief pilot can tell CFIs to avoid flying low altitude maneuvers over the town even though it’s not clearly marked on the sectional chart.

In our area we have a number of homes and farms we know to avoid because low altitude maneuvers cause concern or consternation with the residents, even though they are perfectly legal places to fly and uncongested areas are hard to find in the SF Bay Area. But in general flight schools do want to find workable solutions to keep neighbors happy, since their business relies on the airport staying open.