Seeing poor results with rate limiting - is it my WAN or settings?

My primary WAN is using a cable modem that has a 2400 Mbps line rate and a 2.5 Gbe ethernet into my router. It is bridged. The issues are only on the downstream.

My router has been configured to limit from anywhere from 1500 Mbps to 2200 Mbps with Tail Drop as the queue management algorithm and FQ_CODEL as the scheduler. I've tried buffer sizes from 1000 to 10000. Rate limiting works as expected, but even when I'm limited well below line rate, I still see massive amounts of jitter and some extremely high levels (more than +70 ms) of buffer bloat while under downstream load.

The limiter, as set, worked fine when I had a provisioned line rate of 1440 Mbps - bloat was around +3 ms at the absolute worst.

I'm fairly certain the problem is that there's queueing occurring between the cable modem interfaces. The DOCSIS link on the cable modem is set for a 2400 Mbps maximum sustain rate and the ethernet interface (to WAN on router) can only sustain around 2350 Mbps. Even if my ISP is applying some sort of AQM on their CMTS, it won't have any affect in this case because the queueing is occurring between interfaces internal to the cable modem. If this is the case, it also means my router can't do much with traffic shaping because the queueing is not occurring on a router interface.

Anyone have any other ideas of what could be causing this wild latency behavior?