I've probably got about a dozen pairs of bib shorts including
Decathlon, Specialized, Polaris, Merlin and Castelli and can say without doubt that the best ones for long distance are the Merlin Elite bib shorts.
In fact, they are so good I now have 4 pairs and at £45 would be very difficult to beat. Even on rides in excess of 100 miles I have never experienced any discomfort.
https://www.merlincycles.com/merlin-wear-elite-cycling-bib-shorts-78605.html
Graham