SweetFX worked fine for me. I had to install it as usual, then copy the _64bit subfolder up one level - then it worked just fine. The default install is 32bit and that does not work with Horizons.
Works great with SweetFx 2.0 / Reshader. Just follow the instructions w.r.t. the 64 bit dll (rename to dxgi.dll), but you also need to copy ReShade.fx and Sweet.fx to the same place and copy the SweetFX folder. It picks up the settings file SweetFx_settings.txt from the SweetFx folder.
maybe a little bit late but I only changed the "32bit" to "64bit" in the config file of EDFX -> Documents -> EDFX(2305) -> edfx.cfg -> open with a Textfile Editor and only change the 32bit into 64 bit and it worked. CMDR Skypat who made this will release a 64 bit version of it soon but this is a workaround!