Not to add more confusion, but have you tried shorts with a gel chamois? I used to have chafing issues on longer rides, tried gel, and it was like night and day. I have several pairs of LG Equipe shorts and men's bibs - all keep me comfortable on 50-100 mile rides. I also have a saddle with a channel, which has improved things overall (you mentioned the SI was a cutout, so probably that's not a factor). Brooks saddles take a really long time to break in (you probably know this), and you said it was new-ish? Maybe it hasn't gotten to the "sweet spot" yet?