Wide tires noticably increase steering effort, as would underinflation
(which you have ruled out). Otherwise, a savvy aligner can do wonders for
you. Understand that there is a trade-off between steering ease and highway
stability, but within those limitations the steering can be lightened
significantly. Ask friends for recommendations, and if they don't have any
ask places on the phone if they can decrease your steering effort. You will
know by what they say if they are up to it.

Mike