Halo 3 is a lot of fun, and it’s got some brilliant setpieces, but in terms of plot it’s always felt a bit of a rush job.
Even so, I love it to bits and it occupied many memorable evenings between 2008-2010. Definitely a much easier Legendary experience than Halo 2.
When I did a legendary Combat Evolved solo run, there were one or two speedrunning strats I borrowed. Probably could do the same for H2 - one of the later levels, you can skip entirely if you do it right.