First, I’m an aggressive trail rider that really moves around on the sled a lot. I do very little off trail.
I switched from bibs to pants last year after I got a Tekvest. I couldn’t get comfortable with the Tekvest and bibs. Tekvest over bibs resulted in a lot of friction of the bibs on my chest – probably because I move around so much. (Irritated chest and sore nipples = no fun) Tekvest under the bibs felt really bulky and constricting. After two days of riding I had to find another solution.
I purchased some nice Ski-Doo snowcross pants on sale, and was soon asking myself why I didn’t switch years ago. Very comfortable and non-constricting all around. Much better ventilation up top when I need it. I’ve never had a problem with drafts or snow up the back or anywhere else, and haven’t felt any significant difference in warmth overall. (Use a Castle X jacket.) I spent a fair amount of time riding around zero degrees and was fine with the pants / jacket. The modular helmet is a different story with two incidents of frostbite. I might have to find another helmet this winter for cold days. As you note, pants are much more comfortable when you stop for meals, restroom, etc.
I can’t vouch for pants for off-trail, but I don’t see ever going back to bibs myself for trail riding.