Send imams, moslems, etc. in the target province some turns before your invasion to prepare religious background
Also in high zeal place you have to have a governor with high piety instead of dread. And many happy buildings of course. Does town guard play such a role too?
But be ready to few revolts in Portugal and other freedom loving provinces anyway.
Also one 100-men unit even of peasants sounds much more then few 60-men units.