Only thing I have in mind.

Double the fares, none of this “free rides” the people using the buses are presumably still working, a little more money from each person to keep the system running.

Also bus drivers can choose if they want to work or not, if they are worried about COVID-19 due to paying at the front they can choose to go unemployed. But the system needs to collect money. The alternative of 70% lay-off and reduced services isn’t worth the trade off.

Install makeshift barriers...

These are desperate times and there only bad choices to make. Trying to look for “perfect” solutions and keep everyone happy is the worst thing we can do, which is doing nothing.