We have a Dometic Sealand TW-Series Macerator and Discharge Pump" model T12 (12 volt). Which, according to the instructions, works with a manual pump toilet, not a vacu-flush. Toilet paper goes into a little stainless trash can with a lid and liner, not into the head.
The macerator was already installed on the boat when we bought it, so it is at least a 2013 model. The original installation instructions have a 2007 date on them.
Just like the OP, we pump from the head into the holding tank, then pump out either at the dock
using the on-deck opening, or discharge via the macerator pump while at sea, following the 3-mile rule
. (The Y-valve remains in the "tank" position at all times.)
Since cruising, we have used the macerator pump at least weekly and haven't had a single
problem with it. Not one. It has worked flawlessly with the flip of a switch. (Knock on wood.) Once the tank is empty, we switch the macerator pump off, then manually pump fresh seawater from the head into the tank to "rinse" the tank, hoses, and macerator, then activate the macerator pump again. On the ocean, we have been able to clearly see the discharge, and rinse until the discharge water
is clear. Total time to empty and rinse the 27 gallon holding tank is about 5-10 minutes.
On a side note, when we repaired a hairline crack in the original fiberglass holding tank, we discovered that the waste exit tube had shifted and was no longer sucking from the bottom of the tank, but instead was about half-way up. When we re-positioned the tube, the tank suddenly had twice the capacity. Imagine that.
Best of luck!