but what if you used tor normally, not through your own bridge, but through "regular" randomly chosen 3-hop circuits and at the same time run a tor relay (entry/middle)
This wouldn't require weakening the tor circuit model anymore. Yes, this is correct. The more Tor traffic goes through the machine that identifies you, the more confused an adversary is. It also makes naïve correlation attacks impossible,⁽¹⁾ and increases cost of more advanced ones.
That is great news mpan, thank you. That would incentivize users to also become relays - why isn't it recommended more often? This is the first time I ever hear about it and it sounds like a powerful idea. Normally I only see tor relay operators claim that they run tor relays purely altruistically: https://www.reddit.com/r/TOR/comments/6znjkg/why_would_anyone_setup_a_tor_re...
To know why Tor Project itself doesn’t speak on this matter, you’d need to wait for a reply from somebody from the project.
I may speculate, that the two topics are orthogonal: running a relay and using Tor. They don’t interfere with each other. In your original question they didn’t either. The problem was not running a relay and using Tor, but using Tor with the number of hops effectively reduced.
It would also be a poor advice, if directed towards a person wishing to only connect to Tor. Running a relay from home isn’t without downsides. Both for the operator (bandwidth use, facing hostility) and the network itself (completely inexperienced person is an easier attack target).
Are you aware of any articles from torproject or research papers confirming that hosting tor relay at your own IP does in fact help your own traffic blend in? I've looked through all tor proposals (https://spec.torproject.org/proposals) and many research papers (https://www.freehaven.net) and couldn't find any mentions of this? Specifically for Tor? No. For exactly the same reason I can’t point
you to any research that confirms, that downloading 500 kB/s and 200 kB/s over Tor requires 700 kB/s. It’s a trivial consequence of basic knowledge for the given field. In this case probabilistics, flavored with practicality of correlation attacks and with signal processing basics (none of this in Tor specifically).