My 2c:
~ you seem to be doing a lot of thing right (I love my T-Cento Bib, also use Assos cream + wash after every use)
~ someone mentioned time on the bike - many hours in the saddle training for a 200-mile-in-one-day ride certainly toughened up my behind. I think it is hours more than distance (albeit there is obviously a connection between the two)
~ I would certainly avoid using the soap the hotel provide in the washroom - I use soap with no fragrence / additives etc (took my own supply for a recent London-Paris trip)
~ I am very careful in rinsing all the soap out of my shorts (but I acknowledge I go overboard in removing soap residue in other situations as well - eg rinsing off washing up)
~ Not clear if you have more than one bib short - I allow longer than overnight to properly dry
~ Do you try relieving 'sit-bone' pressure on a frequent basis - eg standing up (even if not necessary from a 'hill' point of view)? For my 200-mile ride I was advised to stand up off the saddle for a short while at regular intervals

fyi I have an SMP Drakon saddle that works for me. But saddles are very personal, what works for one will not work for another. Saddles can also be time-consuming and possibly expensive to resolve - I would be sure to eliminate other possible causes (eg see above for my contribution) before getting into saddles

Good luck -