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.
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. (...) The problem was not running a relay and using Tor, but using Tor with the number of hops effectively reduced.
True, I believe the original question is solved now - as you pointed out my proposal of self-hosting and using a guard node from your public IP would put it at a risk of de-anonymization to various attacks that tor network and many new proposals tries hard to avoid (vanguards in arti is a great example). You rightly mentioned that guard node is intentionally set for longer periods to make it less likely for a malicious relay to be chosen.
There were two aspects to this question. 1. whether it's a de-anonymization risk, which you solved. 2. whether hosting tor traffic of other users around your public IP will help you blend in and strengthen your anonymity. While it makes sense to me and I believe to other users as well if performed via a separate relay, but I would prefer to find more third party academic source, ideally from torproject itself to confirm that.