We’ve released “Append Features to Layer” v1.2 for QGIS!
In this version of the plugin you can use the processing algorithms via command line (qgis_process)!
$ qgis_process run "etl_load:appendfeaturestolayer" -- SOURCE_LAYER=/tmp/source.shp TARGET_LAYER=/tmp/target.shp ACTION_ON_DUPLICATE=0
See the different use cases for “Append Features to Layer”: https://github.com/gacarrillor/AppendFeaturesToLayer/
Thanks to Andreas Rabe for the contribution.