I wonder if eventually we could sidestep the use of bactiophages and instead manufacture the microscopic structures themselves as sunscreen.

There’s a good number of biological processes that are much simpler, cheaper, and require much less materials when the biological process is preserved. A good example of this is water cleaning/breaking down sewage with bacteria which give off methane which is also collected as fuel. Given that the main outcome here is sunscreen that doesn’t damage biology and it’s generally not that expensive to keep sustain life like this, it might make the most sense to simply leave it at production/farming of bacteriophages.