Mixnet would be a solution. Like what you described, have inbound packets held for some period of time and released as a group so that you cannot as easily correlate the inbound and outbound traffic.

The downside is that it gets much slower, and feels 'bad' as an end user. Each packet takes longer.

The only solution I know of is essentially to do "bandwidth burning" where you inject a bunch of fake traffic as noise. I don't know how you'd do that within the constraints of this system.