qgis - Automating GIS slope and aspect algorithm?

I am currently obtaining slope and aspect of roofs using ArcGIS as follows:

1. Mosaic several Lidar .asc files to new raster


2. Extract by mask using Mastermap building outlines to get hold of just the building pixels.


3. Spatial Analyst Tools – Surface – Slope


4. Spatial Analyst Tools – Zonal – Zonal Statistics as Table - mean

I have a large amount of data to process. I could automate this using ModelBuilder in ArcGIS but I’m sure this is not the most efficient method of achieving the results I need. I’m considering the following:

1. Script everything in Python


2. Combination of Python/QGIS/GRASS


3. R software



4. Python/Saga combination

Does anyone have any experience of a similar problem and how did they solve it?

Does anyone have an opinion on which software or language I should select?