The incentive can be very easily removed by legislating that a part-time job would need to offer a fraction of the benefits proportional to the amount of hours the employee works per week.
Ideally, there should be some sort of a credit transfer mechanism, where an employee working multiple part-time jobs that combine to 40+ hours a week could get benefits equivalent to a full-time job at one of the workplaces, proportionally covered by all employers.
Another option would be to crack down on monopolies and large chains. If you employ 10000 corporate drones, who are expected to follow the same corporate instructions day-to-day and can be swapped out by a fresh hire without any loss of productivity, they by definition don't have any economic leverage. On the other hand, if the same market niche is occupied by 1000 independent 10-employee companies, the overall efficiency is lower, but the leverage of each employee (and the salary they can negotiate through supply/demand) is higher. Because in a smaller company you are someone the owner trusts to delegate part of their business, and not just a line on the spreadsheet.