With a 2019 MBP, are there any benefits to driving a 4K display via Displayport (1.4) vs HDMI (2.0)?
No

Freesync sometimes isn't supported over HDMI, but this monitor supports it. On top of that, macOS doesn't support Freesync anyways.

USB-C can "natively" use "normal" DisplayPort and HDMI in its alternate modes, so as long as your dock supports HDMI 2.0, there should be no differences. (N.b.: do note that this is is different from DisplayPort++, but practically speaking, this doesn't really matter for your purpose.)