If the assignment really only takes 30 minutes, why can't it be a live call if the candidate wants?
whether you like it or not, take home assignments just select for the people who can spend the most time on take home assignments.
Which means people like me who have a current job and a family and can't afford to spend several hours on a coding test to make sure they're above and beyond the competition either won't apply, or will send in results that will be much often worse than the competition because we actually only spent 30 minutes on it.