A lot of media coverage, especially in elections has to do with expectations. Biden is an incumbent facing no real opposition in the primary. Trump had real opposition, and there was a chance he would lose. You could argue he’s a semi-incumbent but I don’t think the media views him that way. Reporting on his overcoming this obstacle is naturally going to look a little more positive. In contrast, Biden has little to no chance of losing but has somehow managed to create major opposition to his candidacy anyway. This is noteworthy.
The non-committed vote is an unusual event and it ties into an important issue: the US government’s ongoing material support for ethnic cleansing in Gaza. I think it would be quite bizarre if this did not get coverage.
I am not saying that arguments of bias are automatically wrong, but as you say they have been (falsely, I think) repeated by every president. It’s going to take some compelling evidence and argumentation to overcome my natural skepticism of this idea. So far, I haven’t seen any real case be made. Not to mention that I think there is generally a greater danger in coverage of the powerful that is too positive as compared to too negative. See right-wing media’s fawning Trump coverage for an example.