If you want to make it singular like he/she/it, then make it singular.
He has a car, she has a car, they has a car.
He was friendly, she was friendly, they was friendly.
He sounds fine, she sounds fine, they sounds fine.
Notice the issue?
A singular they is an okay concept, but you then have actually allow it to be singular, in every use - a direct replacement for he/she with no other word or sentence changes necessary.