I've also pondered about custom functions to rank each commit. Maybe you can mark some sub-directory of the repo as 8x more likely to cause trouble than others

I've also pondered about custom functions to rank each commit. Maybe you can mark some sub-directory of the repo as 8x more likely to cause trouble than others and that would effect the split point decision. For example if this seems like a UI bug mark the frontend/ directory as extra suspicious. But don't completely rule out the backend as it may be triggering the frontend bug.