I also sit on the ramus inferioris bones as I find it way more comfortable and stable than sitting on the ischial tuberosities. I can't say I've encountered the problems the OP has. The pad does get narrower toward the front, but so do the bones, so it all lines up quit well.

I can't help with short/bib recommendations as I use mostly cheap big-store stuff, but I wanted to chime in just to let the OP know it's perfectly fine to sit forward of the ischial tuberosities and that they should change their shorts and not their posture.