I agree with the others that your bibs are going to be the most comfortable thing. Having said that, wearing a shell over bibs tends to move around, slip down, and snag the nose of the saddle. MTB shorts with a liner usually have little connectors on the sides to keep the two attached. Many are also elastic or have a belt. You spend a LOT more time out of the saddle on an MTB ride. For two hours or less, you can get away with the inferior mtb chamois if you want to test a cheap integrated combo (like ZOIC).
Another option is the Rapha shorts with pockets on the legs to get the best of both worlds.
I tend to wear all 3 of these options depending on the day. They all work.
At least where I live, if you make MTB friends, they will definitely give you a ribbing if you don’t wear a shell you fru-fru Roadie! The solution is to drop them on the climbs and flats! 