He did trust, thing is that you're confusing faith with stupidity, from what sanji knew at that time, in order to save him, luffy would've had to fight big mom's entire crew while having kaido on his back, that's not something even Roger could do
Sanji giving up on his dream and going back to a life of abuse and trauma took an insane amount of will
And sanji just wanted to keep luffy safe, and as I said, how is willingly walking towards you death not brave?
Sanji made though decisions that took a lot of will power and bravery
Zoro himself thought sanji made the right choice since he also thinks the crew stands no chance against both big mom and kaido
Sanji didn't want to burden the crew, that's a brave decision