Its hard to pin point that any part of you set is is obviously wrong.
I use the
Decathlon cheapo shorts for evening training rides of up to a couple of hours without any problem. Similarly your saddle should be okay ridden starkers, let alone with Assos cream!
So, perhaps you need to look at the detail first. How many miles have you got into your Brookes? How many miles has your arse got recently? You are 'commando' in your shorts aren't you ? Are there any obvious stitching faults in the shorts? Not unknown at this end of the market. Have you tried altering the tilt of your saddle? Very slightly down at the nose is the perceived wisdom although some find others alignments also comfortable.
If you are sure you have eliminated any likely problems with your present set up, then perhaps a better pair of shorts would be worth it on a longer tour.
Finally (and this is more for information than to criticise you choice) the more upright position on a hybrid does put more load on your arse. Although hybrids are sold 'for comfort', the traditional touring bike with the load shared more evenly between the seat and arms, still offers many people a more comfortable long distance option.
PS: If the rubbing is on the inner thigh, some people find they need to slightly pare down the lower edge of the sides of the B17 where it is rubbing. A very sharp craft knife can be used to take off a slither.