I would recommend that you create a template context contributor to
inject either a "helper" class or your service to the
templates instead of enabling service locator.
https://help.liferay.com/hc/en-us/articles/360018170931-Template-Context-Contributor
Using a "helper class" (usually I call it SomethingManager)
has several advantages. You can fetch data from multiple services and
combine them. You also can enrich the data from the service with extra
information or prepare it for easier use in the frontend. This allows
you to write rather clean freemarker code without a lot of logic.
It is also better from a security point of view to avoid enabling
service locator. In the last years the use of Service Locator was
restricted more and more (which is a good thing, IMHO, but of course,
it forces you to settle your technical debt in that regard and convert
everything to context contributors)
