I just realized something, I forgot about pause frames. The modem can tell your WAN NIC to back off, which will give time for packets to buffer in your firewall. This will give some decent benefit, but it will not stop the buffer bloat issue.
Personally, I disabled pause frames because of these issues they can cause, but they're fine for point-to-point interfaces, like your WAN into your modem.
In my case, pause frames makes pretty much no difference because my ISP recently changed our ONTs to run at full 1Gb, then they traffic shape upstream. I used to get a hard stop at my max rate, but now it has a slight burst to it. Unless I attempt to transfer 1Gb/s, I won't get pause frames.