In principle no problem. BUT, if I understand correctly, you are wearing one pair of trousers, then you carry four more pairs and possibly there is a sixth pair of trousers included in what you call a casual outfit.

I'll be a little cocky here just because it's so easy, sorry. But do you count on soiling yourself or why do you need 5-6 pairs of trousers for a five day trip?

If you want to take all that and put the entire Brenthaven bag, that won't work. Go down to two pants packed and one pair on your body. Wear the blazer because the airboss is not great for suit jacket packing in my eyes, others might have different opinions.

Stuff underwear and socks in your shoes. Don't use a bulky toiletry kit but only ziploc bags, one for the liquids and one for the accessories like hair brush etc.

This way you should be good to go and might even be able to fit the Brenthaven bag inside the airboss. Did you check the outer measures of the Brenthaven case?

Till